                 Cite as: 555 U. S. ____ (2008)            1

                   ROBERTS, C. J., dissenting

SUPREME COURT OF THE UNITED STATES
         PENNSYLVANIA v. NATHAN DUNLAP
  ON PETITION FOR WRIT OF CERTIORARI TO THE SUPREME 

      COURT OF PENNSYLVANIA, EASTERN DISTRICT

             No. 07–1486. Decided October 14, 2008

   The petition for a writ of certiorari is denied.
   CHIEF JUSTICE ROBERTS, with whom JUSTICE KENNEDY
joins, dissenting from denial of certiorari.
   North Philly, May 4, 2001. Officer Sean Devlin, Narcot­
ics Strike Force, was working the morning shift. Under­
cover surveillance. The neighborhood? Tough as a three­
dollar steak. Devlin knew. Five years on the beat, nine
months with the Strike Force. He’d made fifteen, twenty
drug busts in the neighborhood.
   Devlin spotted him: a lone man on the corner. Another
approached. Quick exchange of words. Cash handed over;
small objects handed back. Each man then quickly on his
own way. Devlin knew the guy wasn’t buying bus tokens.
He radioed a description and Officer Stein picked up the
buyer. Sure enough: three bags of crack in the guy’s
pocket. Head downtown and book him. Just another day
at the office.
                        *    *     *
  That was not good enough for the Pennsylvania Su­
preme Court, which held in a divided decision that the
police lacked probable cause to arrest the defendant. The
Court concluded that a “single, isolated transaction” in a
high-crime area was insufficient to justify the arrest, given
that the officer did not actually see the drugs, there was
no tip from an informant, and the defendant did not at­
tempt to flee. 941 A. 2d 671, 679 (2007). I disagree with
that conclusion, and dissent from the denial of certiorari.
A drug purchase was not the only possible explanation for
the defendant’s conduct, but it was certainly likely enough
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                   ROBERTS, C. J., dissenting

to give rise to probable cause.
   The probable-cause standard is a “nontechnical concep­
tion that deals with the factual and practical considera­
tions of everyday life on which reasonable and prudent
men, not legal technicians, act.” Maryland v. Pringle, 540
U. S. 366, 370 (2003) (internal quotation marks omitted).
What is required is simply “a reasonable ground for belief
of guilt,” id., at 371 (same)—a “probability, and not a
prima facie showing, of criminal activity,” Illinois v. Gates,
462 U. S. 213, 235 (1983) (same). “[A] police officer may
draw inferences based on his own experience in deciding
whether probable cause exists,” Ornelas v. United States,
517 U. S. 690, 700 (1996), including inferences “that might
well elude an untrained person,” United States v. Cortez,
449 U. S. 411, 418 (1981).
   On the facts of this case, I think the police clearly had
probable cause to arrest the defendant. An officer with
drug interdiction experience in the neighborhood saw two
men on a street corner—with no apparent familiarity or
prior interaction—make a quick hand-to-hand exchange of
cash for “ ‘small objects.’ ” 941 A. 2d, at 673. This ex­
change took place in a high-crime neighborhood known for
drug activity, far from any legitimate businesses. Perhaps
it is possible to imagine innocent explanations for this
conduct, but I cannot come up with any remotely as likely
as the drug transaction Devlin believed he had witnessed.
In any event, an officer is not required to eliminate all
innocent explanations for a suspicious set of facts to have
probable cause to make an arrest. As we explained in
Gates, “[i]n making a determination of probable cause the
relevant inquiry is not whether particular conduct is
‘innocent’ or ‘guilty,’ but the degree of suspicion that at­
taches to particular types of noncriminal acts.” 462 U. S.,
at 244, n. 13.
   The Pennsylvania Supreme Court emphasized that the
police did not actually see any drugs. 941 A. 2d, at 679.
                 Cite as: 555 U. S. ____ (2008)           3

                   ROBERTS, C. J., dissenting

But Officer Devlin and his partner were conducting un­
dercover surveillance. From a distance, it would be diffi­
cult to have a clear view of the small objects that changed
hands. As the Commonwealth explains in its petition for
certiorari, the “classic” drug transaction is a hand-to-hand
exchange, on the street, of cash for small objects. Pet. for
Cert. 5–8. The Pennsylvania Supreme Court’s decision
will make it more difficult for the police to conduct drug
interdiction in high-crime areas, unless they employ the
riskier practice of having undercover officers actually
make a purchase or sale of drugs.
   The Pennsylvania Court also noted that the defendant
did not flee. 941 A. 2d, at 671. Flight is hardly a prereq­
uisite to a finding of probable cause. A defendant may
well decide that the odds of escape do not justify adding
another charge to that of drug possession. And of course
there is no suggestion in the record that the defendant had
any chance to flee—he was caught redhanded.
   Aside from its importance for law enforcement, this
question has divided state courts, a traditional ground
warranting review on certiorari. S. Ct. Rule 10(b). The
New Jersey Supreme Court has held that an “experienced
narcotics officer” had probable cause to make an arrest
when—in a vacant lot in a high-drug neighborhood—he
“saw defendant and his companion give money to [a] third
person in exchange for small unknown objects.” State v.
Moore, 181 N. J. 40, 46–47, 853 A. 2d 903, 907 (2004). The
Rhode Island Supreme Court reached the same conclusion
in a case where the defendants—through their car win­
dows—exchanged cash for a small “bag of suspected nar­
cotics.” State v. Castro, 891 A. 2d 848, 851–854 (2006). In
contrast, the Colorado Supreme Court held that a hand-to­
hand exchange of unknown objects did not give the police
probable cause to make an arrest, even where one of the
men was a known drug dealer. People v. Ratcliff, 778
P. 2d 1371, 1377–1378 (1989). All these cases have unique
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                   ROBERTS, C. J., dissenting

factual wrinkles, as any probable-cause case would, but
the core fact pattern is the same: experienced police offi­
cers observing hand-to-hand exchanges of cash for small,
unknown objects in high-crime neighborhoods.
   The Pennsylvania Supreme Court speculated that such
an exchange could have been perfectly innocent. But as
Judge Friendly has pointed out, “[j]udges are not required to
exhibit a naiveté from which ordinary citizens are free.”
United States v. Stanchich, 550 F. 2d 1294, 1300 (CA2
1977). Based not only on common sense but also his experi­
ence as a narcotics officer and his previous work in the
neighborhood, Officer Devlin concluded that what happened
on that street corner was probably a drug transaction. That
is by far the most reasonable conclusion, even though our
cases only require it to be a reasonable conclusion.
   I would grant certiorari and reverse the judgment of the
Pennsylvania Supreme Court.
