Science News - April 24, 2010

vaccination side effects and are befuddled by parents’ unfounded autism concerns.

Decision making based on experiences such as this one is beginning to draw intense scientific interest. New work probes how personal experiences twist risk perceptions differently from assessments that include probabilities. Evidence from gambling games suggests that decisions based on personal experience may actually benefit from peoples’ limited memories. A third line of investigation explores how disaster exposures in different countries shape people’s sensitivity to human fatalities and their willingness to endorse risky public health programs. And scientists are examining the nature of decisions that blend experience with secondhand descriptions, with an eye toward improving the effectiveness of federal medication warnings and product recalls.

Paradoxical terror

For more than 30 years, psychologists have used gambling games to chart people’s tendency to overestimate the chances of hitting the jackpot, or losing big, when given exact probability figures.

But Israeli college students’ seemingly incompatible reactions to a string of suicide bombings highlight the limits of that approach, says psychologist Greg Barron of Harvard University. The bombings occurred on 71 days between September 30, 2000, and August 31, 2002.

When tested in the weeks after bombings had been publicly called off by Palestinian leaders, a group of 43 students reported taking special precautions on days after attacks with fatalities. Students’ cautious behavior reflected an experience-based overestimation of the probability that bombers would immediately strike again, as happened on 17 percent of days following deadly attacks. In a life-or-death situation, such overestimates make sense from a safety standpoint.

Despite the behavior exhibited by the first group, another group of similar students reported believing that the chances of another suicide attack were less the day after a fatal attack than after a quiet day. In fact, bombers struck on

only 9 percent of days following a quiet day, suggesting students underplayed the chances of history immediately repeating itself when asked to provide an absolute risk estimate.

“Overweighting and underestima-tion of rare events apparently coexist in the same individuals,” Barron says. These paradoxical results, published in the October 2009 Judgment and Decision Making, contradict a long-standing scientific assumption that probability estimates — although often incorrect — guide real-life gambles.

Barron’s study, conducted with Eldad Yechiam of Technion–Israel Institute of Technology in Haifa, also challenges a popular view that people underestimate the risk of a rare event mainly because they can remember only a few recent, remotely relevant personal experiences. In fact, Israeli students had plenty of vivid memories of recent suicide bombings.

Contrasting intuitions about the causes of sequences of events may have shaped the students’ assessments of risks, Barron suggests. A focus on attacks as the intentional acts of human agents may have created an anticipation that bombers would immediately strike again, explaining cautious behavior, he proposes. That’s akin to what’s called the hot-hand fallacy, in which observers believe that, say, a basketball player who makes several shots in a row will make the next shot because he’s “in the zone.” In contrast, an impersonal focus on the probability of another bombing the day after a fatal attack could have prompted students to appeal to their understanding of random chance. Previous studies suggest that people regard chance as a process ensuring that whichever of two possibilities last happened will likely change the next time around, Barron says. That corresponds to the gambler’s fallacy, in which, say, a roulette player believes the ball is bound to land on black after settling on red several times in a row.

Much remains unknown about condi-
tions that encourage the hot-hand or the
gambler’s fallacy, remarks psychologist
Ralph Hertwig of the University of Basel
in Switzerland. “I know of no data on how
many people who go to casinos accept the
gambler’s fallacy or the extent to which
either of these effects differ across indi-
viduals,” he says.

Sampling power

Hertwig suspects that people flexibly adopt different decision-making strategies to deal with life’s shifting, ambiguous risks. In support of that possibility, sev-

Coexisting concepts in fall 2000, a period of palestinian suicide bombing in israel began (car explosion in 2001 shown). a study found that the behavior of israeli students reflects an overestimation of the risk of a bombing on a day following a fatal attack, while students’ probability estimates underplay the risk of a bombing immediately after a fatal attack.