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.
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.
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.
Reported cautiousness among students
( 1 to 5 scale)
Perceived risk of a suicide bombing
Percent of days on which
suicide bombing occurred
( 1 to 5 scale)
Actual risk of a suicide bombing
source: g. barron and e. yechiam/Judgment and