Interrogate suspects separately and get them to incriminate one another—that's how cops do it. New research suggests that a better way to catch colluding criminals might be to interview them together.

In a recent experiment in the Journal of Applied Research in Memory and Cognition with more than 40 pairs of subjects, half were told to steal 10 and then convince an interviewer of their innocence. The other pairs were told the money had gone missing. The truth tellers interrupted one another four times as often and were much more likely to add to or correct their friend's account. The liars said less and hardly interrupted.

The deceivers were not simply more taciturn, however. “It's a myth to think suspects are reluctant to talk,” says social psychologist Aldert Vrij of the University of Portsmouth in England. “Mainly U.S. police manuals promote this myth.” Further, in the antiterrorism situations for which Vrij's research is designed, a “no comment” could lead a person to be taken off a plane or denied entry to a country. Because liars must talk in such situations, an interviewer who paid attention to a pair's interruptions and contradictions might better tell truth from fiction than one seeking only suspicious silent types. [For more on how to spot a scoundrel, see page 70.]

A simple, straightforward narrative only hints at falsehoods when suspects share a cover story, Vrij says. One truth teller and one liar in a pair will act differently from either two liars or two truth tellers.

Because many terrorist acts are planned by groups, this finding suggests group interviews can be a useful tool for law enforcement and border patrols. As Vrij explains, “truth tellers' interaction with one another comes naturally and is not natural for liars.”