 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued November 14, 2018           Decided September 13, 2019

                        No. 18-3017

                UNITED STATES OF AMERICA,
                       APPELLANT

                              v.

   JOSEPH RICKY PARK, ALSO KNOWN AS JOSEPH DEMASI,
                       APPELLEE


        Appeal from the United States District Court
                for the District of Columbia
                   (No. 1:16-cr-00009-1)


    Sonja M. Ralston, Attorney, U.S. Department of Justice,
argued the cause and filed the briefs for appellant. Elizabeth
Trosman, Assistant U.S. Attorney, entered an appearance.

    A.J. Kramer, Federal Public Defender, argued the cause
and filed the brief for appellee. Tony Axam Jr. and Celia
Goetzl, Assistant Federal Public Defenders, entered
appearances.

    Before: GARLAND, Chief Judge, and GRIFFITH and
PILLARD, Circuit Judges.

    Opinion for the Court filed by Circuit Judge PILLARD.
                               2
    Opinion concurring in part and concurring in the judgment
by Circuit Judge GRIFFITH.

     PILLARD, Circuit Judge: One hundred and seventy-six
nations have come together to develop a coordinated, global
approach to fight the sexual exploitation of children. Building
on the United Nations Convention on the Rights of the Child,
all but a handful of the nations of the world have agreed to the
Optional Protocol on the Sale of Children, Child Prostitution
and Child Pornography (Optional Protocol or Protocol). Its
signatories jointly committed to take many common steps to
protect children, including criminalizing various child sex
offenses. Optional Protocol, art. 3. The Protocol also
empowers its signatories to police their own nationals’ sexual
exploitation of children wherever it takes place. Id. art. 4. The
United States Senate ratified the Protocol in 2002. Among the
laws that fulfill the United States’ duties under the Protocol is
the Prosecutorial Remedies and Other Tools to End the
Exploitation of Children Today Act, 18 U.S.C. § 2423 (2018)
(PROTECT Act).

    Defendant Joseph Ricky Park is a U.S. citizen with a prior
conviction of a sex offense against a minor in the United States.
Park now faces a PROTECT Act indictment for further sex
crimes against a minor in Vietnam. He challenges Congress’s
constitutional authority to criminalize what he is alleged to
have done in a foreign country.

     Park was convicted in Connecticut of child sexual abuse
decades ago. Since the 1990s, Park has for the most part been
traveling and living abroad, including in Mexico, Cuba, South
Korea, the Philippines, Thailand, Russia, Kuwait, China, Laos,
Singapore, Malaysia, Saudi Arabia, Bahrain, Lebanon,
Cambodia, and Vietnam. As Park traveled the world, he
worked as an English teacher and, the government contends,
                               3
sexually abused children. He often moved from one country to
the next once local law enforcement authorities suspected him
of child sex abuse.

     The United States apprehended Park in 2016 and indicted
him for producing child pornography and sexually abusing a
child while residing in Vietnam in 2015, in violation of the
PROTECT Act, 18 U.S.C. §§ 2423(c), (e), (f)(3), (f)(1).
Vietnam is a signatory to the Optional Protocol. Vietnam
appears to have cooperated in Park’s apprehension and raises
no objection to the United States prosecuting him here on
charges arising out of his conduct in Vietnam. Park
successfully moved the district court to dismiss the indictment
on the ground that Congress lacks constitutional authority for
the application of a federal criminal prohibition to child sexual
abuse and production of child pornography in a foreign
country. The government appeals.

     We hold that the PROTECT Act is constitutional as
applied to Park. Each of the provisions that Park challenges is
rationally related to implementing the Optional Protocol, a
treaty of unchallenged validity to which the United States and
Vietnam are signatories. The provisions of the PROTECT Act
that criminalize child sexual abuse and production of child
pornography by U.S. citizens living abroad help to fulfill the
United States’ responsibility under the Optional Protocol to
criminalize, “as a minimum,” child prostitution and child
pornography production by U.S. nationals wherever that
conduct occurs. Optional Protocol, arts. 3, 4. Congress’s
authority under the treaty to prosecute U.S. citizens’
extraterritorial crimes involving sexual exploitation of children
is bolstered by the Foreign Commerce Clause, which supports
application of U.S. law to economic activity abroad that, in the
aggregate, could otherwise impair the effectiveness of a
                              4
comprehensive regulatory regime to eliminate the sexual
exploitation of children.

                    I. BACKGROUND

A. Factual Background

     When reviewing the grant of a motion to dismiss an
indictment, we accept the government’s factual allegations as
true. See Boyce Motor Lines v. United States, 342 U.S. 337,
343 & n.16 (1952); United States v. Ballestas, 795 F.3d 138,
149 (D.C. Cir. 2015). The government has provided additional
information regarding Park’s conduct in Vietnam and history
of sexually abusing minors, and Park concedes that we
appropriately assume the truth of that information as well.
Appellee Br. 9-10.

     Park has sexually abused minors in multiple countries over
the past thirty years. In 1987, the State of Connecticut
convicted Park of two counts of “Risk of Injury to a Child” and
one count of “Sexual Assault 2nd Degree.” Appendix for the
United States (U.S. App.) 10; Park Appendix (Park App.) 24.
After serving five years in prison, Park traveled to Mexico
where, the United States alleges, he sexually abused children.
Mexico extradited Park to the United States in 1995, and
Connecticut re-imprisoned him until 1998 for violating the
terms of his probation. In 2003, Park traveled from the United
States to Cuba, where the Cuban government arrested and
incarcerated him for nearly three years for “Corruption of a
Minor.” Park App. 24-25. Park again left the United States
and, in 2009, South Korean authorities revoked his visa and
ordered him to leave that country after they received
information that he engaged in “indecent behavior” while
working there as a schoolteacher. U.S. App. 11. In 2013, Park
was apparently teaching English in Saudi Arabia when he was
asked to leave because of his “pedophile” behavior. U.S. App.
                               5
14. Park went to Vietnam, where he remained on short-term
tourist and business visas until November 2015.

     The crimes for which the government now charges Park
allegedly took place in 2015, while Park was working as an
English teacher in Vietnam. The government alleges that Park
introduced himself to an eleven-year-old Vietnamese boy (the
alleged victim) at a park in Hanoi. Park told the boy he was an
English language instructor and invited him to his apartment
for lessons. Several weeks later, that same boy and two others
visited Park at his apartment. The young friends and Park
played a game that involved “chasing and grasping” each other,
including on Park’s bed. U.S. App. 12-13. The three boys then
played videogames in Park’s bedroom, with the victim sitting
on Park’s lap. Park began to “pinch” and stroke the boy’s
genitals through his clothing, telling the victim that he, Park,
“wanted to make him feel good.” Id. at 13. Park then tried to
reach his hand inside the boy’s pants, but the boy pushed Park’s
hand away.

     The victim’s mother reported Park’s conduct to the United
States Department of State, and Vietnam deported Park to
Thailand. While in Thailand, Park asked a friend in Hanoi to
collect various belongings from his apartment. The friend
discovered child pornography on Park’s computer and thumb
drive and turned the evidence over to United States special
agents. A forensic review later confirmed that the devices
contained child pornography depicting unidentified victims in
videos produced from July 2013 through August 2015.

     In early 2016, Park left Thailand for Guam where a special
agent for the United States Department of Homeland Security
arrested him on January 15. A federal grand jury indicted Park,
based on his conduct while residing in Vietnam, for violating
18 U.S.C. §§ 2423(c) and (e), which criminalize actual or
                                 6
attempted “illicit sexual conduct with another person” engaged
in by “[a]ny United States citizen . . . who travels in foreign
commerce or resides, either temporarily or permanently, in a
foreign country.” Park moved for a bill of particulars, and the
government specified that the charged “illicit sexual conduct”
involved “the actual and attempted production of child
pornography” and “an actual and attempted sexual act as
defined in 18 U.S.C. Section 2246.” See Gov’t Resp. to Def.’s
Bill of Particulars 2. The pornography charges are based only
on images produced after May 30, 2015, which is when
Congress amended the PROTECT Act to include production of
child pornography.

B. The Optional Protocol and the PROTECT Act

     The Optional Protocol seeks to end the sexual exploitation
of children by committing the countries of the world to
eradicate child sex trafficking, child prostitution, and child
pornography. The Protocol expresses “[d]eep[] concern[] at
the widespread and continuing practice of sex tourism, to
which children are especially vulnerable, as it directly
promotes the sale of children, child prostitution and child
pornography.” Optional Protocol, preamble. Under article 3,
paragraph 1, the parties to the Protocol (States Parties) have
obligated themselves to criminalize, “as a minimum,” acts of
“[o]ffering, delivering or accepting . . . a child for the purpose
of [s]exual exploitation of the child,” “obtaining . . . a child for
child prostitution,” and           “[p]roducing,      distributing,
disseminating, importing, exporting, offering, selling or
possessing for the above purposes child pornography,” whether
those offenses are committed “domestically or transnationally
or on an individual or organized basis.” Id. art. 3(1). The
Protocol further authorizes each State Party to exercise
jurisdiction over persons who violate the article 3, paragraph 1
prohibitions “[w]hen the alleged offender is a national of that
                                7
State or a person who has his habitual residence in its territory.”
Id. art. 4(2)(a).

     As noted, both the United States and Vietnam are
signatories to the Optional Protocol, which was adopted and
opened for signature in May 2000 and went into effect in
January 2002. President Clinton signed the Protocol in July
2000. See Letter of Submittal from President Clinton to the
Department of State, Protocols to the Convention on the Rights
of the Child, S. Treaty Doc. No. 106-37, 2000 WL 33366017,
at *1 (July 5, 2000) (Protocol Analysis). The President’s letter
transmitting the treaty to the Senate urged its consent because
participation in the Protocol would “enhance the ability of the
United States to provide global leadership in the effort to
eliminate abuses against children” that involve “sexual
exploitation.” Letter of Transmittal from President Clinton to
the Senate of the United States, Protocol Analysis at *1. The
Senate consented to the ratification on June 18, 2002. See 148
Cong. Rec. S5717-01 (daily ed. June 18, 2002). Vietnam
signed the Protocol in September 2000 and ratified it in
December 2001.

     The commercial sexual exploitation of children, which
includes both child pornography and international child sex
tourism, has grown rapidly over the past two decades into a
multibillion-dollar industry. See Najat Maalla M’jid, Report of
the Special Rapporteur on the Sale of Children, Child
Prostitution and Child Pornography, U.N. Doc. A/HRC/22/54,
at 9 (Dec. 24, 2012) (2012 U.N. Report); see also United States
v. Durham, 902 F.3d 1180, 1195 (10th Cir. 2018), cert. denied,
139 S. Ct. 849 (2019). Child sexual abuse images alone have
been estimated to be worth about 20 billion dollars. 2012 U.N.
Report at 9. And “[m]any developing countries have fallen
prey to the serious problem of international sex tourism,” yet,
“for reasons ranging from ineffective law enforcement, lack of
                                8
resources, corruption, and generally immature legal systems,
sex tourists often escape prosecution in the host countries.”
H.R. Rep. No. 107-525, at 2-3 (2002). Travelers seek out
children to sexually abuse in a shifting subset of countries
where they anticipate lax law enforcement: “As child
protection laws, mechanisms and prevention efforts are
strengthened by States, civil society and the tourism industry
in some countries, neighboring countries become obvious
alternative destinations for travelling sex offenders.” 2012
U.N. Report at 5. Affected countries often “reach out to the
United States for help,” and “some even blame the United
States for the problem, arguing that many of the sex tourists are
American.” H.R. Rep. No. 107-525, at 3. For this reason,
people around the world have historically looked to the United
States to take responsibility to put an end to sex offenders’
abuse of children overseas. Id.

     Congress proposed the Sex Tourism Prohibition
Improvement Act of 2002 to address “this growing problem.”
Id. The House Judiciary Committee reported on the proposed
bill and recommended it for passage just six days after the
Senate consented to the ratification of the Optional Protocol.
Id. at 1. Renamed the PROTECT Act, the bill was signed into
law a year later. Pub. L. No. 108-21, 117 Stat. 650 (2003); see
generally United States v. Bollinger, 798 F.3d 201, 208 (4th
Cir. 2015). The PROTECT Act amended an existing, narrower
federal criminal bar against travel undertaken with the intent to
commit illicit sexual conduct. See 18 U.S.C. § 2423(b) (2000).
In light of experience showing that it was difficult to prove that
travel was undertaken with the requisite intent, the new statute
included a provision reaching sex crimes committed by U.S.
citizens abroad without regard for the initial purpose of the
international travel. See H.R. Rep. 107-525, at 3; 148 Cong.
Rec. H3885 (daily ed. June 25, 2002) (statement of Rep.
Sensenbrenner, co-sponsor).
                                 9
     Passage of the PROTECT Act did not eliminate
misgivings about the adequacy of the United States’
implementation of the Protocol. The United Nations, for
instance, repeatedly expressed its “concern” that the United
States’ extraterritorial jurisdiction “did not reach all offenses
covered by the Optional Protocol” and urged the United States
to “establish its jurisdiction in all cases listed under article 4 of
the Optional Protocol,” which calls on parties to assume
jurisdiction over their nationals for all article 3, paragraph 1
offenses (which, as noted above, include sexual exploitation
and child pornography). Comm. on the Rights of the Child,
Concluding Observations on the Second Periodic Report of the
United States of America Submitted Under Article 12 of the
Optional Protocol to the Convention on the Sale of Children,
Child Prostitution and Child Pornography, ¶¶ 39-40, U.N.
Doc. CRC/C/OPSC/USA/CO/2 (July 2, 2013) (2013
Concluding Observations); Comm. on the Rights of the Child,
Consideration of Reports Submitted by States Parties Under
Article 12, Paragraph 1, of the Optional Protocol to the
Convention on the Rights of the Child on the Sale of Children,
Child Prostitution and Child Pornography, ¶¶ 35-36, U.N.
Doc. CRC/C/OPSC/USA/CO/1 (June 25, 2008) (2008
Consideration of Reports). U.S. courts also expressed concern
that the PROTECT Act’s prohibition on “travel[ing] in foreign
commerce[] and engag[ing] in any illicit sexual conduct,” 18
U.S.C. § 2423(c) (2006), might not reach U.S. citizens who had
settled abroad. See, e.g., United States v. Schmidt, 845 F.3d
153, 156-58 (4th Cir. 2017); United States v. Jackson, 480 F.3d
1014, 1022-1024 (9th Cir. 2007).

    Meanwhile, it had become apparent that “known child-sex
offenders [were] traveling internationally.” International
Megan’s Law to Prevent Child Exploitation and Other Sexual
Crimes Through Advanced Notification of Traveling Sex
Offenders, Pub. L. No. 114-119, § 2(4), 130 Stat. 15, 15 (2016)
                              10
(International Megan’s Law). Before 2016, the federal Sex
Offender Registration and Notification Act (SORNA), 42
U.S.C. § 16901 et seq., did not require sex offenders to update
their location information in the sex offender registry system
when they traveled or moved abroad, and sex offenders took
advantage of that loophole by relocating to foreign countries
without giving notice to federal authorities. See, e.g., Nichols
v. United States, 136 S. Ct. 1113, 1118 (2016); United States v.
Lunsford, 725 F.3d 859, 861-62 (8th Cir. 2013). Offenders
gravitated to certain developing countries, such as the
Philippines, which were known to have “significant problems
with sex tourism.” H.R. Rep. No. 107-525, at 3.

     Congress responded to these concerns by amending
section 2423(c) in 2013 to reach illicit sexual conduct by U.S.
citizens and permanent residents who “reside[], either
temporarily or permanently, in a foreign country.” Violence
Against Women Reauthorization Act of 2013, Pub. L. No. 113-
4, Title XII, § 1211(b), 127 Stat. 54, 142 (2013). And, in May
2015, Congress added “production of child pornography” to
the definition of “illicit sexual conduct” in section 2423(f).
Justice for Victims of Trafficking Act of 2015, Pub. L. No.
114-22, Title I, § 111(a)(3), 129 Stat. 227, 240 (2015).

    In relevant part, the PROTECT Act now reads:

       (c) Engaging in illicit sexual conduct in foreign
       places.—Any United States citizen or alien
       admitted for permanent residence who . . .
       resides, either temporarily or permanently, in a
       foreign country, and engages in any illicit
       sexual conduct with another person shall be
       fined under this title or imprisoned not more
       than 30 years, or both.

       ...
                               11
       (e) Attempt and conspiracy.—Whoever
       attempts or conspires to violate subsection . . .
       (c) . . . shall be punishable in the same manner
       as a completed violation of that subsection.

       (f) Definition.—As used in this section, the term
       “illicit sexual conduct” means—

               (1) a sexual act (as defined in section
               2246) with a person under 18 years of
               age that would be in violation of chapter
               109A if the sexual act occurred in the
               special maritime and territorial
               jurisdiction of the United States;

               ...

               (3) production of child pornography (as
               defined in section 2256(8)).

18 U.S.C. § 2423. As relevant here, section 2246 defines the
term “sexual act” to include the “intentional touching, not
through the clothing, of the genitalia of another person who has
not attained the age of 16 years with an intent to abuse,
humiliate, harass, degrade, or arouse or gratify the sexual desire
of any person.” Id. § 2246(2)(D).

     Park’s is one of the first prosecutions brought under either
the new “resides” language in section 2423(c), added in 2013,
or the child-pornography production language in section
2423(f), added in 2015.

C. The Decision of the District Court

    The district court dismissed the indictment on February 28,
2018, on the ground that the Foreign Commerce Clause, the
                               12
treaty power, and Congress’s inherent, plenary powers over
foreign affairs all failed to authorize the application of 18
U.S.C. § 2423(c) to the conduct for which Park was indicted.
United States v. Park, 297 F. Supp. 3d 170, 174-83 (D.D.C.
2018).

     The district court held that Congress’s treaty power did not
support section 2423(c) on the facts of this case. The court
doubted the PROTECT Act implemented the Optional Protocol
because the Act’s legislative history was “devoid” of any
indication that Congress acted with such a purpose. Id. at 180.
Even if Congress enacted section 2423(c) to implement the
Protocol, the court viewed the statute, as applied to Park’s non-
commercial conduct, as not rationally related to the “single
goal of the Optional Protocol,” which was, according to the
court, “to address the States Parties’ grave concerns regarding
the ‘international traffic of children.’” Id. (emphasis in
original) (quoting Optional Protocol, preamble). The court also
concluded that the Foreign Commerce Clause did not authorize
Congress to reach Park’s conduct. Applying to the foreign
commerce power the three-part Interstate Commerce Clause
framework established in United States v. Lopez, 514 U.S. 549
(1995), the court held that it could not sustain the Act’s
application here under any part. Park, 297 F. Supp. 3d at 174-
79. Finally, because of “the absence of case law supporting the
government’s position” on the point, the court declined to hold
that “Congress has plenary powers over citizens and foreign
affairs that empower it to act” beyond the scope of an
enumerated constitutional power. Id. at 183.
                               13
                        II. ANALYSIS

    The government argues on appeal that Congress’s treaty
power and the Foreign Commerce Clause support the
application of 18 U.S.C. § 2423 to Park’s conduct in Vietnam.
Accordingly, we must determine whether the PROTECT Act,
as applied to Park, is a “necessary and proper means to”
implement the Optional Protocol, Missouri v. Holland, 252
U.S. 416, 432 (1920), or whether it falls within the scope of
Congress’s foreign commerce powers. Our review is de novo.
See Hodge v. Talkin, 799 F.3d 1145, 1155 (D.C. Cir. 2015);
McKesson Corp. v. Islamic Republic of Iran, 539 F.3d 485, 488
(D.C. Cir. 2008).

     We start from the premise that “the ‘question of the
constitutionality of action taken by Congress does not depend
on recitals of the power which it undertakes to exercise.’” Nat’l
Fed’n of Indep. Bus. v. Sebelius, 567 U.S. 519, 570 (2012)
(quoting Woods v. Cloyd W. Miller Co., 333 U.S. 138, 144
(1948)). A court must be able to discern a basis for Congress’s
exercise of an enumerated power, but that does not mean that a
“law must be struck down because Congress used the wrong
labels” or failed to identify the source of its power. Id. at 569-
70. As a practical matter, congressional findings may help a
court to understand a statute’s operation, such as by explaining
its connection to commerce. Gonzales v. Raich, 545 U.S. 1,
20-21 (2005). But so long as there is no ground for heightened
judicial scrutiny of its action—none is asserted here—
Congress need not make “particularized findings in order to
legislate.” Id. at 21 (distinguishing Turner Broadcasting
System, Inc. v. FCC, 512 U.S. 622, 664-68 (1994)).

    Congress’s power to legislate may also stem from more
than one enumerated power. See United States v. Morrison,
529 U.S. 598, 607 (2000) (noting that “[e]very law enacted by
                                 14
Congress must be based on one or more of its powers
enumerated in the Constitution”); see also United States v.
Lara, 541 U.S. 193, 200-01 (2004) (noting that Congress’s
“plenary and exclusive” power to legislate with respect to
Indian tribes derives from both the Indian Commerce Clause
and the power to implement treaties); Legal Tender Cases, 79
U.S. (12 Wall) 457, 534 (1870) (noting that it is “allowable to
group together any number of [the specified constitutional
powers] and infer from them all that the power claimed has
been conferred”); McCulloch v. Maryland, 17 U.S. (4 Wheat.)
316, 407 (1819) (noting that several of Congress’s powers
support creation of a national bank). Where, as here,
Congress’s treaty and Commerce Clause powers dovetail, both
powers may provide support for the constitutionality of
Congress’s actions, see Lara, 541 U.S. at 200-02, which in our
view makes it appropriate to examine all potential sources, but
see Concurring Op., infra.

A. Congress’s treaty power reaches Park’s conduct.

     Article II of the Constitution empowers the President to
make treaties with the advice and consent of the Senate. U.S.
Const. art. II, § 2, cl. 2. The Necessary and Proper Clause, U.S.
Const. art I, § 8, cl. 18, in turn, confers on Congress the “power
to enact such legislation as is appropriate to give efficacy to . . .
treat[ies]” made by the President with the advice and consent
of the Senate. Neely v. Henkel, 180 U.S. 109, 121 (1901). In
Justice Holmes’s memorable formulation, “[i]f the treaty is
valid there can be no dispute about the validity of the statute
under Article 1, Section 8, as a necessary and proper means to
execute the powers of the Government.” Holland, 252 U.S. at
432. Congress’s power to enact legislation it deems necessary
and proper to implement a valid treaty is commonly referred to
as the “treaty power.” Lara, 541 U.S. at 201.
                               15
     “[I]n determining whether the Necessary and Proper
Clause grants Congress the legislative authority to enact a
particular federal statute, we look to see whether the statute
constitutes a means that is rationally related to the
implementation of a constitutionally enumerated power.”
United States v. Comstock, 560 U.S. 126, 134 (2010) (citing
Sabri v. United States, 541 U.S. 600, 605 (2004)). The inquiry
is “simply ‘whether the means chosen are “reasonably adapted”
to the attainment of a legitimate end.’” Id. at 135 (quoting
Raich, 545 U.S. at 35 (Scalia, J., concurring in judgment)). In
this case, the “legitimate end” is implementation of the
Optional Protocol. If it is apparent that the means Congress has
chosen are “convenient, or useful, or conducive” to effectuate
a valid treaty, id. at 134-35 (quoting McCulloch, 17 U.S. (4
Wheat.) at 413), then “the degree of their necessity, the extent
to which they conduce to the end, the closeness of the
relationship between the means adopted and the end to be
attained, are matters for congressional determination alone,”
id. at 135 (quoting Burroughs v. United States, 290 U.S. 534,
548 (1934)).

     Accordingly, to determine whether the challenged
provisions as applied to Park are within the scope of federal
authority, we consider whether they are rationally related to
implementing the Optional Protocol’s goals. These goals
include not only, as the district court observed, 297 F. Supp. 3d
at 180, combating the “international traffic of children,” but
also “eliminat[ing] . . . child prostitution and child
pornography,” and addressing international “sex tourism,”
Optional Protocol, preamble. Because the government charged
Park with only one count, which encompasses both Park’s child
pornography production and child sex abuse, the indictment
stands so long as Congress had the authority to reach either
type of conduct. We hold that both applications are
constitutionally valid exercises of Congress’s treaty power.
                              16
Each of the provisions under which he is charged—
criminalizing production of child pornography by a U.S. citizen
residing abroad, 18 U.S.C. §§ 2423(c), (f)(3), and non-
commercial child sexual abuse by a U.S. citizen residing
abroad, id. §§ 2423(c), (f)(1)—helps to eradicate the sexual
exploitation of children that the Optional Protocol targets.
Each provision is therefore rationally related to fulfilling the
United States’ obligations under the treaty.

   1. The PROTECT Act’s prohibition against United
      States citizens producing child pornography while
      residing abroad is rationally related to
      implementing the Optional Protocol.

     The PROTECT Act’s prohibition against U.S. citizens
producing child pornography while residing abroad rationally
relates to two aspects of the Optional Protocol. First, the
Optional Protocol requires the States Parties to criminalize the
production of child pornography. Second, it empowers them
to exercise jurisdiction over the pertinent offenses of their
nationals regardless of where the offenses occur. The Protocol
thus constitutionally supports indictment of Park, a U.S.
citizen, for producing child pornography in Vietnam.

     The Optional Protocol directs the States Parties to
criminalize the production of child pornography. Each State
Party “shall prohibit . . . child pornography as provided for by
the present Protocol,” Optional Protocol, art. 1, including
specifically prohibiting the “[p]roducing, distributing,
disseminating, importing, exporting, offering, selling or
possessing for the above purposes child pornography,” id.
art. 3(1)(c). By criminalizing the “production of child
pornography” by U.S. citizens abroad, 18 U.S.C. § 2423(f)(3),
the PROTECT Act is rationally related to implementing the
Optional Protocol.
                               17
     Park objects that the Optional Protocol is concerned only
with commercial child pornography, so the PROTECT Act’s
ban on child pornography homemade for one’s own use, not
bought or sold—i.e., the type of conduct alleged against Park—
is not rationally related to the implementation of the Protocol.
The Protocol is not so confined. It calls on States Parties to
prohibit the production of child pornography without limitation
to any proven commercial conduct or plans.

     “When interpreting a treaty, we begin with the text of the
treaty and the context in which the written words are used,”
applying all “general rules of construction” to aid our
understanding. E. Airlines, Inc. v. Floyd, 499 U.S. 530, 535
(1991) (internal quotation marks and citation omitted). The
preamble to the Optional Protocol states an ultimate goal of
“elimination of . . . child pornography,” without limitation to
commercially traded images, such that even non-commercial
production falls within its scope. Optional Protocol, preamble.
The Optional Protocol also capaciously defines “child
pornography” as “any representation, by whatever means, of a
child engaged in real or simulated explicit sexual activities or
any representation of the sexual parts.” Id. art. 2(c).

     The States Parties chose not to limit the Optional Protocol
to commercial child pornography production for obvious
reasons. As a practical matter, the line between possession of
and trade in pornographic images is exceedingly fine and
fragile. “[C]hild pornography is now traded with ease on the
Internet” and, in the digital age, “the number of still images and
videos memorializing the sexual assault and other sexual
exploitation of children, many very young in age, has grown
exponentially.” Paroline v. United States, 572 U.S. 434, 440
(2014) (quoting Patti B. Saris et al., U.S. Sentencing Comm’n,
Federal Child Pornography Offenses 3 (2012)). Child
pornography stored online can be distributed worldwide almost
                               18
instantaneously. United States v. Sullivan, 451 F.3d 884, 891
(D.C. Cir. 2006).

     Commercial transactions in child pornography can be
difficult if not impossible to establish where no traceable
payment means is used. Child pornography producers have
strong incentives to barter their images, leaving no monetary
transaction record. Indeed, online child-pornography groups
often require people seeking to join to upload new images in
order to gain access to images already contributed by existing
members. Regardless of whether child pornography is created
with any commercial purpose, production of digital
pornographic images of children expands the stock of such
images worldwide. The ease with which they may be
converted to commercial use means that, if left unregulated,
non-commercial production of such images could substantially
hinder efforts to eliminate the international child pornography
market. Once an image is uploaded, each new transfer
“multipl[ies] the existing supply of the commodity, so that even
if the initial possessor’s holdings are destroyed, subsequent
possessors may further propagate the images.”                  Id.
Criminalizing production only where there is proof of a
monetary transaction or commercial purpose would be a mere
half measure toward halting the supply of child pornography
available to the illegal market, and so fall short in serving one
of the primary purposes of the treaty: “the elimination . . . of
child pornography.” Optional Protocol, preamble.

     That the treaty requires the criminalization of
“[p]roducing,     distributing,     disseminating,     importing,
exporting, offering, selling or possessing for the above
purposes child pornography,” id. art. 3(1)(c) (emphasis added),
does not, as Park suggests, limit its terms to child pornography
produced for commercial distribution. He reads the phrase “for
the above purposes” as confined to either the other “purposes”
                                19
expressly identified in Article 3—“sexual exploitation of the
child,” “transfer of organs of the child for profit,” or
“engagement of the child in forced labor,” id. art. 3(1)(a)(i)—
or the general activities listed in subsections (a) and (b) of
Article 3—the sale of children and child prostitution, id. art.
3(1)(a), (b). However, we typically apply the “rule of the last
antecedent” when interpreting a text that “include[s] a list of
terms or phrases followed by a limiting clause.” Lockhart v.
United States, 136 S. Ct. 958, 962 (2016). Thus, “a limiting
clause or phrase . . . should ordinarily be read as modifying
only the noun or phrase that it immediately follows.” Barnhart
v. Thomas, 540 U.S. 20, 26 (2003). As used here, the phrase
“for the above purposes” modifies only the last antecedent,
“possessing,” and references the listed purposes of “producing,
distributing, disseminating, importing, exporting, offering,
[and] selling” child pornography. UNICEF adopts this reading,
in fact recognizing it as the one most protective of potential
offenders: “Interpreted strictly, article 3(1)(c) of the [Protocol]
obliges States Parties to punish the possession of child
pornography only when this possession is ‘for the above
purposes’—producing, distributing, disseminating, importing,
exporting, offering or selling.” UNICEF, Handbook on the
Optional Protocol on the Sale of Children, Child Prostitution,
and Child Pornography 12 (2009); see also id. (noting that the
“Committee on the Rights of the Child has nevertheless
encouraged countries to prohibit simple possession”). Given
its narrow scope, the phrase “for the above purposes” in no way
limits to commercial production the Protocol’s prohibition
against “producing” child pornography.

     Because the Optional Protocol, by its terms, reaches both
commercial and non-commercial production of child
pornography, the PROTECT Act’s criminalization of non-
commercial child pornography production plainly implements
the treaty and is constitutional as applied to Park.
                               20
     Congress’s decision to apply the PROTECT Act to
Americans who “reside[], either temporarily or permanently, in
a foreign country,” 18 U.S.C. § 2423(c), similarly fulfills the
Optional Protocol’s expectation that States Parties will take
jurisdiction over the misdeeds of their nationals wherever they
occur.

     The Optional Protocol reflects agreement that each State
Party “may take such measures as may be necessary to
establish its jurisdiction” over offenses “[w]hen the alleged
offender is a national of that State.” Optional Protocol, art.
4(2). This type of jurisdiction, where a country prescribes law
with respect to the “conduct, interests, status, and relations of
its nationals and residents outside its territory,” is known as
“active personality jurisdiction” or “nationality jurisdiction.”
Restatement (Fourth) of Foreign Relations Law of the United
States, § 402(1)(c), cmt. g & rep. note 7 (Am. Law Inst. 2018).
Under international law, every nation has “jurisdiction over its
subjects travelling or residing abroad, since they remain under
its personal supremacy,” and the United States is no exception.
Blackmer v. United States, 284 U.S. 421, 437 n.2 (1932)
(quoting L. Oppenheim, 1 International Law 281 (4th ed.
1926)). Congress retains authority over U.S. citizens residing
abroad “[b]y virtue of the obligations of citizenship.” Id. at
436; accord United States v. Curtiss-Wright Exp. Corp., 299
U.S. 304, 318 (1936).

     When the United States originally ratified the Protocol,
however, it chose not to exercise its nationality jurisdiction
over its citizens’ conduct abroad. See Protocol Analysis at *23.
The United Nations twice criticized the United States for that
reticence, stressing that the United States must “establish its
jurisdiction in all cases listed under article 4” of the Optional
Protocol in order to “strengthen the framework for prosecution
and punishment.” 2013 Concluding Observations ¶¶ 39-40;
                                21
2008 Consideration of Reports ¶¶ 35-36. Congress could have
rationally concluded that, to fully implement the United States’
obligations under the Protocol, it needed to respond to
international opprobrium by expanding the coverage of section
2423(c) to criminalize child pornography produced by U.S.
citizens residing abroad. Indeed, in 2016, the United States
cited the revised version of section 2423(c), reaching offenses
by U.S. citizens residing abroad, as evidence of its continuing
efforts to fulfill its responsibilities under the Optional Protocol.
See Dep’t of State, Combined Third and Fourth Periodic
Report of the United States of America on the Optional
Protocols to the Convention on the Rights of the Child on the
Involvement of Children in Armed Conflict and the Sale of
Children, Child Prostitution, and Child Pornography, ¶ C-57
(Jan. 22, 2016).

     Park objects that the PROTECT Act does not implement
the Optional Protocol because, in his view, the “Protocol ‘does
not require the United States to criminalize the production of
child pornography in another country.’” Appellee Br. 50
(quoting Park, 297 F. Supp. 3d at 181) (emphasis in Park). He
contends that the Optional Protocol addresses only child
pornography produced domestically within the United States
or produced “transnationally,” which he somewhat awkwardly
reads to mean “between the United States and another nation.”
Id. at 45 (quoting Optional Protocol art. 3(1)).            But
“transnationally” is often used to mean simply “reaching
beyond national boundaries,” see, e.g., Philip Jessup,
Transnational Law 2 (1956) (defining “transnational law” to
“include all law which regulates actions or events that
transcend national frontiers”); Transnational, Black’s Law
Dictionary (2019) (defining “transnational” as “[i]nvolving
more than one country”). The Protocol’s coverage of both
domestic and transnational offenses is naturally read as
exhaustive, encompassing, for example, both what a citizen of
                                22
one country does within his own country and what he does
abroad. Indeed, this reading accords with the view of the
United Nations itself, which has observed that
“[e]xtraterritorial legislation is one of the key tools in
combating [child sex tourism], as it allows legal authorities to
hold nationals and citizens accountable for crimes committed
abroad.” 2012 U.N. Report at 11. The full text of the sentence
Park quotes shows an intent to sweep broadly. In requiring
States Parties to criminalize the specified conduct whether it is
“committed domestically or transnationally or on an individual
or organized basis,” Optional Protocol, art. 3(1), the treaty calls
for bans on that conduct no matter where it is committed, or by
one person or many. The PROTECT Act’s prohibition on the
production of child pornography by U.S. citizens abroad is
rationally related to the implementation of this final clause.

      Moreover, it is unlikely that the States Parties intended
Park’s crabbed and ineffectual reading, which would
criminalize domestic and “transnational” activity but not the
acts of U.S. citizens within foreign countries. A “treaty is a
contract . . . between nations,” and its “interpretation normally
is, like a contract’s interpretation, a matter of determining the
parties’ intent.” BG Grp., PLC v. Republic of Argentina, 572
U.S. 25, 37 (2014). Here, the text itself encourages the States
Parties to go further than its bare terms. The same sentence on
which Park relies also states that “[e]ach State Party shall
ensure that, as a minimum” the conduct described is
criminalized. Optional Protocol, art. 3(1). The preamble to the
Optional Protocol further recognizes that “the elimination of
the sale of children, child prostitution and child pornography”
would require “a holistic approach.” Id., preamble. Where the
text of a treaty “create[s] a floor, not a ceiling” in this manner,
Congress may properly implement the treaty’s intent by going
further in its implementing legislation. United States v. Belfast,
611 F.3d 783, 807 (11th Cir. 2010). Accordingly, the
                                23
“extraterritorial application” of the PROTECT Act, 18 U.S.C.
§ 2423(c), to Park’s conduct while he was residing abroad is
expressly permitted by the Optional Protocol.

    2. The PROTECT Act’s prohibition of child sexual
       abuse by United States citizens residing abroad is
       rationally related to implementing the Optional
       Protocol.

    The Optional Protocol prohibits the “[o]ffering, obtaining,
procuring or providing a child for child prostitution,” Optional
Protocol art. 3(1)(b), and defines “child prostitution” as “the
use of a child in sexual activities for remuneration or any other
form of consideration,” id. art. 2(b). As such, the Protocol does
not itself specifically address non-commercial child sexual
abuse. Nevertheless, the PROTECT Act’s broader prohibition
on child sex abuse by U.S. citizens residing abroad, including
non-commercial crimes, 18 U.S.C. §§ 2424(c), (f)(1), was
appropriate to combat commercial child sex tourism and
control the problem of American sex offenders relocating and
sexually abusing children abroad, thereby closing enforcement
gaps that otherwise could have hindered the objectives of the
Optional Protocol.

      The Necessary and Proper Clause empowers Congress to
fill “regulatory gaps” that could otherwise be left by its exercise
of constitutionally enumerated legislative powers. Sabri v.
United States, 541 U.S. 600, 607 (2004); see United States v.
Kebodeaux, 570 U.S. 387, 395 (2013). Here, the Optional
Protocol’s goal of eliminating commercial child sexual
exploitation, including global sex tourism, could be undercut if
Congress failed to criminalize non-commercial child sex abuse
by U.S. residents abroad. This is so for at least three reasons.

    First, as a general matter, such a “loophole in the law”
could encourage American sex tourists—who by some
                              24
estimates comprise one quarter of all sex tourists globally—to
go abroad seeking non-commercial sex with minors that, had it
occurred in the United States, would be criminalized as
statutory rape. “If Americans believe that traveling to a
particular foreign country includes the opportunity for
unregulated, non-commercial illicit sexual conduct, they may
travel to that country when they otherwise would not . . . .”
United States v. Lindsay, 931 F.3d 852, 863 (9th Cir. 2019);
see also United States v. Pendleton, 658 F.3d 299, 311 (3d Cir.
2011). The “Constitution does not envision or condone” such
“a vacuum” of power in which “citizens may commit acts
abroad that would clearly be crimes if committed at home.”
Bollinger, 798 F.3d at 219.

     Second, and relatedly, Congress might well have
concluded that the PROTECT Act’s prohibition of non-
commercial sexual exploitation of minors by U.S. residents
abroad was appropriate to ameliorate a specific externality of
the United States’ intensified domestic policing of child sexual
abuse: the relocation to other countries of registered U.S. sex
offenders and the risks such offenders may pose there. Until
2016, SORNA did not require registered sex offenders in the
United States to update their sex offender registrations when
they moved abroad. See, e.g., Nichols, 136 S. Ct. at 1118.
Consequently, “known child-sex offenders [were] traveling
internationally,” International Megan’s Law § 2, and some
relocated abroad to get out from under SORNA’s registration
requirements, see, e.g., Nichols, 136 S. Ct. at 1117-18;
Lunsford, 725 F.3d at 861-62. (After the events at issue here,
Congress took further steps to address this externality,
amending the law to require registered U.S. sex offenders to
update their SORNA registrations when they plan to travel
outside the United States, see 34 U.S.C. § 20914(a)(7); 18
U.S.C. § 2250(b).) When domestic legislation creates or
exacerbates identified risks to treaty partners—e.g. when
                               25
domestic counter-recidivism measures like SORNA lead U.S.-
citizen sex offenders to move overseas and commit the very
crimes the Protocol aims to eliminate—Congress’s treaty
power authorizes it to address that danger.

     Third, Congress rationally could have concluded that the
Optional Protocol’s goal of eliminating global sex tourism
involving minors would be undermined unless putatively non-
commercial sex with minors were also criminalized. Congress
was well aware that the quid-pro-quo in child prostitution is
typically more indirect or hidden than for prostitution involving
adults. If a U.S. national could travel overseas and entice a
child with inchoate favors, valuable experiences, promised
future benefits, meals, or other gifts—any of which might be
difficult to establish as “consideration” in support of a child
prostitution      charge—deterrents        against       traveling
internationally to sexually abuse children would be
significantly weakened. The statutory prohibition against non-
commercial child sex abuse is therefore a “vital component” in
the “PROTECT Act’s larger scheme” to “curb the supply and
demand in the sex tourism industry.” Durham, 902 F.3d at
1214.

     Congress’s power to give the treaty practical effect against
conduct like Park’s is not confined to the Optional Protocol’s
minimum requirements. Again, the Protocol identifies the
child sexual exploitation it targets and specifies “a floor, not a
ceiling” on how signatories should address such exploitation
by their nationals abroad. See Belfast, 611 F.3d at 807; United
States v. Lue, 134 F.3d 79, 84 (2d Cir. 1998). The States Parties
to the Optional Protocol recognized that the “elimination of . . .
child prostitution” would require national lawmakers to take “a
holistic approach, addressing the contributing factors,”
including “irresponsible adult sexual behaviour.” Optional
Protocol, preamble. The treaty therefore stipulates that
                               26
criminalizing the conduct it identifies is “only a ‘minimum’
requirement.” Bollinger, 798 F.3d at 219 (quoting Optional
Protocol art. 3). In view of the Protocol’s purpose and scope,
it was reasonable for Congress in enacting the PROTECT Act
“to determine that the non-commercial abuse of children is a
factor that contributes to commercial sexual exploitation, and
to regulate non-commercial conduct accordingly.” Id. And it
was therefore constitutional for Congress to reach Park’s
alleged conduct in this case.

     Our conclusions regarding the treaty power comport with
the fundamental constitutional principle that Congress may
legislate only within the scope of its constitutionally conferred
powers. The government may not simply point to any
tangentially related treaty to defend a constitutionally suspect
statute. There are at least two recognized limits to what
Congress may legislate in the name of implementing a treaty.
First, to be a valid exercise of the Necessary and Proper Clause,
the treaty itself must be “legitimate,” and the statute must be
“plainly adapted to” the treaty. McCulloch, 17 U.S. (4 Wheat.)
at 421. Second, implementing legislation must be both “not
prohibited” by the Constitution and “consistent with the letter
and spirit of the Constitution.” Id. It is “well established that
‘no agreement with a foreign nation can confer power on the
Congress, or on any other branch of Government, which is free
from the restraints of the Constitution.’” Boos v. Barry, 485
U.S. 312, 324 (1988) (quoting Reid v. Covert, 354 U.S. 1, 16
(1957)). Though this inquiry is deferential, it is not toothless.
Here, the PROTECT Act is plainly necessary and proper to
implement the goals of the Optional Protocol.

     Park argues for an additional limit. He claims that we must
first assess “whether a statute is in fact implementing
legislation,” and argues that “§ 2423(c), originally and as
amended, contains no indication that it is implementing the
                                27
Protocol.” Appellee Br. 38. To the extent any such nexus is
required—and Park provides no support for this proposition—
we find it satisfied here. The House Judiciary Committee
recommended passage of what became the PROTECT Act just
six days after the Senate ratified the Optional Protocol. And,
as discussed, Congress passed later amendments to the
PROTECT Act to address loopholes in the international
regulatory scheme.

      In addition, Park passingly suggests that Congress’s treaty
power is confined to helping the President make treaties, and
that “[o]nce a treaty has been made, Congress’s power to do
what is ‘necessary and proper’ to assist the making of treaties
drops out of the picture.” Id. at 37 (quoting Bond v. United
States, 572 U.S. 844, 876 (2014) (Scalia, J., concurring in the
judgment)). According to that view, Congress “must rely upon
its independent . . . Article I, § 8, powers” in order to “legislate
compliance with the United States’ treaty obligations.” Bond,
572 U.S. at 876. But under Missouri v. Holland, 252 U.S. at
433-34, that is not the law. Under long established treaty
power doctrine, the PROTECT Act is constitutional as applied
to Park’s conduct abroad.

    B. Congress’s power under the Foreign Commerce
       Clause further supports application of the
       PROTECT Act to Park.

     Although Congress’s treaty power suffices to support the
constitutionality of the PROTECT Act’s application to Park,
the Foreign Commerce Clause provides further constitutional
authority to criminalize Park’s child pornography production
and non-commercial child sex abuse abroad. Cf. Lara, 541
U.S. at 200 (resting Congress’s “plenary and exclusive” power
to legislate with respect to Indian tribes on both the treaty
power and the Indian Commerce Clause).
                                28
     Article I of the Constitution empowers Congress to
“regulate Commerce with foreign Nations, and among the
several States, and with the Indian Tribes.” U.S. Const. art. I,
§ 8, cl. 3. Because the Foreign Commerce Clause appears
alongside the Interstate Commerce Clause, courts have
generally applied some version of the Supreme Court’s
Interstate Commerce Clause analytic framework to Foreign
Commerce Clause cases. In re Sealed Case, No. 14-3043, 2019
WL 4123971, at *6 (D.C. Cir. June 25, 2019); cf. Gibbons v.
Ogden, 22 U.S. (9 Wheat.) 1, 74 (1824) (noting that the word
“commerce” “must carry the same meaning throughout the
sentence, and remain a unit, unless there be some plain
intelligible cause which alters it”). Under that framework,
Congress may regulate “three broad categories of activity.”
Lopez, 514 U.S. at 558. First, Congress may regulate the “use
of the channels of interstate commerce.” Id. Second, Congress
may regulate the “instrumentalities of interstate commerce, or
persons or things in interstate commerce.” Id. Finally, and
most relevant here, Congress may regulate “those activities . .
. that substantially affect interstate commerce,” id. at 558-59,
so long as the regulated activity is “some sort of economic
endeavor,” Morrison, 529 U.S. at 611.

     With respect to the final category, the Court has clarified
that Congress may also “regulate purely intrastate activity that
is not itself ‘commercial,’ in that it is not produced for sale,” if
Congress “concludes that failure to regulate that class of
activity would undercut the regulation of the interstate market
in that commodity.” Raich, 545 U.S. at 18. A reviewing court
“need not determine whether [challengers’] activities, taken in
the aggregate, substantially affect interstate commerce in fact,
but only whether a ‘rational basis’ exists for so concluding.”
Id. at 22. For this reason, the Court in Raich sustained the
application of federal criminal law to local, non-commercial
production and use of marijuana where Congress had grounds
                              29
to conclude that failure to do so would leave a “gaping hole” in
a comprehensive drug-regulation statute. Id.; see also Wickard
v. Filburn, 317 U.S. 111, 125 (1942) (holding that “even if
appellee’s activity be local and though it may not be regarded
as commerce, it may still, whatever its nature, be reached by
Congress if it exerts a substantial effect on interstate
commerce”).

     We also adapt the Interstate Commerce Clause framework,
and turn to the preliminary question of whether Congress’s
foreign commerce power is broader, narrower, or coextensive
with its interstate commerce power. Park reads relative
restriction in the Foreign Commerce Clause’s grant of power
“to regulate Commerce with foreign Nations,” in contrast to the
domestic clause’s reference to regulation of commerce “among
the several States.” U.S. Const. art. I, § 8, cl. 3 (emphases
added); see also United States v. Al-Maliki, 787 F.3d 784, 793
(6th Cir. 2015) (expressing “skeptic[ism]” that “Congress has
greater commerce power over conduct occurring in foreign
countries than conduct occurring in the States”).

     We disagree with Park’s reading of the Foreign Commerce
Clause as narrower than its domestic counterpart. Indeed, the
point was implicitly rejected by the Supreme Court when it
recognized that Congress’s foreign commerce powers are, at
least in some ways, broader than Congress’s interstate
commerce powers. The Court has noted that “there is evidence
that the Founders intended the scope of the foreign commerce
power to be [] greater” than its interstate counterpart, Japan
Line, Ltd. v. Los Angeles Cty., 441 U.S. 434, 448 & n.13
(1979), and that Congress’s commerce power “when exercised
in respect of foreign commerce may be broader than when
exercised as to interstate commerce,” Atl. Cleaners & Dyers,
Inc. v. United States, 286 U.S. 427, 434 (1932).
                                30
     More explicitly, the Supreme Court has held that the
Indian Commerce Clause, which also uses the preposition
“with” in authorizing Congress to “regulate Commerce . . . with
the Indian Tribes,” U.S. Const. art. I, § 8, cl. 3 (emphasis
added), confers on Congress “plenary power to legislate in the
field of Indian affairs,” Lara, 541 U.S. at 200 (quoting Cotton
Petroleum v. New Mexico, 490 U.S. 163, 192 (1989)). The
Court has characterized the foreign commerce power as at least
equally broad. See Buttfield v. Stranahan, 192 U.S. 470, 493
(1904) (declaring that the “power to regulate foreign commerce
is certainly as efficacious as that to regulate commerce with the
Indian tribes”).

     These conclusions make good sense.                In foreign
commerce, the federalism constraints that limit Congress’s
interstate commerce power are absent, and there is a greater
need for the United States to speak with a single voice. See,
e.g., Japan Line, 441 U.S. at 448 (quoting Bd. of Trs. v. United
States, 289 U.S. 48, 59 (1933)). Unencumbered by federalism
limitations, Congress acts with the full regulatory power of a
nation-state when it legislates regarding its “commerce” with
foreign nations. Therefore, we follow our sister circuits in
adopting an “effects test” that is at least as expansive as the test
employed in the Interstate Commerce Clause context. See
Sealed Case, 2019 WL 4123971, at *6; see also, e.g., United
States v. Durham, 902 F.3d 1180, 1192-93 (10th Cir. 2018)
(“substantial effect”); Pendleton, 658 F.3d at 308 (“substantial
effect”); United States v. Baston, 818 F.3d 651, 668 (11th Cir.
2016) (“substantial effect”); Bollinger, 798 F.3d at 215-16
(“demonstrabl[e] [e]ffect[]”).         We once again find it
unnecessary to “define today the precise level of ‘effect’
necessary,” because the alleged facts taken as true place Park’s
conduct abroad “within Congress’s reach under any version of
the test.” Sealed Case, 2019 WL 4123971, at *6. We hold that
the Foreign Commerce Clause provides constitutional support
                              31
for the challenged indictment of Park for violating the
PROTECT Act.

    1.   The PROTECT Act’s prohibition against United
         States citizens producing child pornography while
         residing abroad is a valid exercise of Congress’s
         foreign commerce power.

     Criminalizing the non-commercial production of child
pornography that affects international commerce is a
straightforward application of Raich and Wickard. The market
effect that supported federal regulation in those cases was the
risk that “production of the commodity meant for home
consumption” could be drawn into the commercial market as a
result of high demand. Raich, 545 U.S. at 19. The same type
of market effect supports the PROTECT Act’s application to
non-commercial child pornography. Both commercial and
non-commercial production may rationally be viewed as
“quintessentially economic” activity, involving the making at
home of a product that presumably would otherwise be
procured in an established illegal market. Sullivan, 451 F.3d at
890-91 (quoting Raich, 545 U.S. at 18). It is reasonable to
conclude that criminalizing the production and possession of
child pornography “increases the ‘price’ of pornography by
attaching the risk of prosecution, a market intervention meant
to eliminate the illicit trade.” Sullivan, 451 F.3d at 891.
Eliminating child pornography is also a reasonable means to
advance Congress’s comprehensive scheme to “eliminate the
market for the sexual exploitative use of children,” because
child sex abusers and traffickers often use child pornography
to “seduc[e] other children into sexual activity.” Omnibus
Consolidated Appropriations Act, Pub. L. No. 104-208,
§ 121(12), (3), 110 Stat. 3009, 3009-26, 3000-27 (1996).
                              32
     Indeed, for some of the same reasons addressed in Part
II.A.1, the effect of non-commercial production of child
pornography on the market for child-pornographic images may
be even greater than the market effects of the commodities at
issue in Raich and Wickard. “The Internet, and its capacity to
facilitate online bartering of computer files between collectors
and purveyors of child pornography, readily links a single
computer user to a possible network of others.” United States
v. Dyer, 589 F.3d 520, 530 (1st Cir. 2009). Pornography
initially produced purely for personal viewing can be used as
intended and also diverted into the market. That is because,
“[i]n contrast to wheat or marijuana, the supply of electronic
images of child pornography has a viral character: every time
one user downloads an image, he simultaneously produces a
duplicate version of that image” meaning that “each new
possessor increases the available supply of pornographic
images.” Sullivan, 451 F.3d at 891. This multiplying effect
“highlights the importance of eliminating a possessor’s stash in
the first instance, before it can be disseminated into the
marketplace.” Id.

     Because electronically stored child-pornographic images
produced for personal use could, in the aggregate, have a
“substantial effect” on national and international markets for
child pornography, it was reasonable for Congress to conclude
that prohibiting non-commercial production of child
pornography abroad was necessary to combat the growing
international market in child pornography. Therefore, as
applied to Park’s conduct, Congress has not exceeded its
constitutional authority.
                               33
   2. The PROTECT Act’s prohibition of United States
      citizens engaging in non-commercial child sex
      abuse abroad is a valid exercise of Congress’s
      foreign commerce power.

     Although the point is a closer one, the PROTECT Act’s
prohibition against U.S. citizens engaging in non-commercial
child sex abuse abroad is also within Congress’s foreign
commerce power. “[N]on-commercial sexual abuse of minors
can drive commercial demand for sex with minors by
reinforcing the idea that such conduct is acceptable, or by
allowing traffickers to use non-commercial arrangements to
entice patrons into engaging in subsequent commercial
behavior.” Lindsay, 931 F.3d at 863. As discussed in detail in
Part II.A.2, leaving such a critical gap could also encourage
U.S. citizens to travel or relocate to foreign countries that do
not, or cannot, successfully police child sexual abuse, thereby
“affect[ing] the price for child prostitution services and other
market conditions in the child prostitution industry.”
Bollinger, 798 F.3d at 219 (quoting United States v. Martinez,
599 F. Supp. 2d 784, 808 (W.D. Tex. 2009)).

     Criminalizing non-commercial sexual abuse is also
conducive to eliminating commercial child exploitation “given
the enforcement difficulties” posed by a requirement to prove
a quid-pro-quo transaction. See Raich, 545 U.S. at 22. Proof
of the commercial aspect of child sexual exploitation can be
exceptionally elusive. International child sex tourists often use
“travel agencies, transport, accommodation and other tourism-
related services that facilitate contact with children,” 2012
U.N. Report at 5, and everyone involved has strong incentives
to disguise their unlawful activities. Tour operators, for
example, charge for seemingly legitimate “fishing trips” as a
cover for arranging sexual access to juveniles. See Appellant
Br. 20 (quoting ECPAT International, Offenders on the Move,
                                 34
Global Study on Sexual Exploitation of Children in Travel and
Tourism 61 (2016)). Some experts report that sexual predators
attempt to gain sexual access to children through a process of
“grooming,” in which the abuser uses “inducements” such as
“money, treats, gifts, [and] fun trips . . . to establish trust, which
then allows the offender to control and manipulate the child
into participating in sexual abuse.” Georgia M. Winters &
Elizabeth L. Jeglic, Stages of Sexual Grooming: Recognizing
Potentially Predatory Behaviors of Child Molesters, 38
Deviant Behav. 724, 726 (2017). The transactional component
of such inducements is systematically denied by and hidden
from the child, the child’s family, and the community, which
makes it challenging for law enforcement to uncover. See id.
at 724-25. Given the nature of commercial child sexual
exploitation, Congress had a rational basis to conclude that a
law requiring proof of commercial activity would result in
dramatic underenforcement.

     To be sure, child sexual abuse may not be “quintessentially
economic” in every case. Cf. Raich, 545 U.S. at 25. Park
therefore argues that such activity is ineligible for aggregation
under the Interstate Commerce Clause framework. Cf.
Morrison, 529 U.S. at 617 (holding that Congress exceeded its
interstate commerce powers in enacting a civil remedy for
victims of gender-motivated violence). But because Park
brings an as-applied challenge, we must assess the “statute’s
constitutionality with respect to the particular set of facts
before [us],” Hodge, 799 F.3d at 1156, and evaluate Park’s
argument in light of the alleged facts that the parties agree we
must take as true.

     Unlike the gender-motivated violence at issue in
Morrison, Park’s alleged acts have features of market-
affecting, transactional economic activity. The government
alleges that Park has traveled throughout the world seeking out
                                 35
opportunities for child sex abuse. In Vietnam, he worked as a
foreign language teacher, offering English language instruction
to gain access to children. Indeed, Park was able to meet and
introduce himself to the specific victim at issue by offering
English lessons. And Park is alleged to have used the prospect
of English lessons to invite the child to his apartment.
Congress had ample reason to conclude that this kind of tacit
and informal exchange to achieve child sexual exploitation,
when aggregated, has a substantial effect on the market for
prostitution and sex trafficking of children.            Without
discounting the possibility that some applications of this statute
may exceed Congress’s authority, we are therefore satisfied on
the facts here that Park’s conduct may be regulated under
Congress’s foreign commerce power.

     Park’s final objection is that applying the “substantial
effects” test in this context permits Congress to act as the
“world’s lawgiver” in derogation of the independent
sovereignty of other nations. See Baston v. United States, 137
S. Ct. 850, 850 (2017) (Thomas, J., dissenting from denial of
certiorari). But the exercise of foreign commerce power here
does not interfere with foreign sovereignty: Congress has
legislated within a consensually established international
regulatory framework, the statute’s extraterritorial reach is
confined to the conduct of U.S. citizens and aliens admitted for
permanent residence, and Vietnam makes no sovereignty-
based objection to Park’s prosecution in the United States (nor
could it). By ratifying the Protocol, Vietnam adopted the
Protocol’s general goal to strive for the “elimination . . . of child
prostitution and child pornography,” Optional Protocol,
preamble, and invited the United States to apply its pertinent
criminal law to conduct by U.S. nationals within Vietnam.
Moreover, “courts have long recognized that Congress may
criminalize many actions that target U.S. nationals or interests
abroad, or provide for the prosecution of much conduct by U.S.
                               36
nationals or organizations overseas.” Sealed Case, 2019 WL
4123971, at *4. However judicially unexplored the outer limits
of Congress’s foreign commerce powers may be, it is clear
that—at least where, as here, there is no interference with
foreign sovereignty—Congress can regulate the economic
activity of U.S. citizens abroad that could, in the aggregate,
undermine a comprehensive international legal regime
designed to prevent transnational exploitation of children.

                           *    *    *

     We conclude that Congress was acting well within its
authority to “make all Laws which shall be necessary and
proper” to carry into effect the federal government’s treaty
power when it enacted the PROTECT Act to implement the
Optional Protocol, an internationally agreed upon regulatory
framework that encourages signatories to assume nationality
jurisdiction over their nationals’ sexual exploitation of children
abroad. Congress’s power to enact the PROTECT Act is
further bolstered by its authority to “regulate Commerce with
foreign Nations.” Therefore, we hold that 18 U.S.C.
§§ 2424(c), (e), (f)(1), and (f)(3) are constitutional as applied
to Park’s indictment. We reverse the district court’s dismissal
of the indictment and remand for further proceedings consistent
with this opinion.

                                                     So ordered.
     GRIFFITH, Circuit Judge, concurring in part and
concurring in the judgment: I agree with my colleagues that the
treaty power authorizes Congress to make criminal Park’s
conduct. See Missouri v. Holland, 252 U.S. 416 (1920).
Although Holland’s premise—that Congress can do by
legislation pursuant to a treaty what it cannot do by ordinary
legislation that reaches beyond its enumerated powers—has
come in for some criticism, see, e.g., Bond v. United States,
572 U.S. 844, 873-82 (2014) (Scalia, J., concurring in the
judgment); id. at 882-96 (Thomas, J., concurring in the
judgment), it remains binding upon this Court and controls the
outcome of this case.

     Because the statute criminalizing Park’s conduct is
constitutional as applied under the treaty power, we have no
need to address the more challenging question of whether the
Foreign Commerce Clause also authorizes Congress to act
here. See Cohen v. Bd. of Trs. of the Univ. of D.C., 819 F.3d
476, 485 (D.C. Cir. 2016) (“[T]he cardinal principle of judicial
restraint—if it is not necessary to decide more, it is necessary
not to decide more—counsels us to go no further.” (quoting
PDK Labs., Inc. v. DEA, 362 F.3d 786, 799 (D.C. Cir. 2004)
(Roberts, J., concurring in part and concurring in the
judgment))); see also Lyng v. Nw. Indian Cemetery Protective
Ass’n, 485 U.S. 439, 445 (1988) (“A fundamental and
longstanding principle of judicial restraint requires that courts
avoid reaching constitutional questions in advance of the
necessity of deciding them.”). For this reason I decline to join
part II.B of the majority opinion.
