“Do you feel sufficiently informed about the importance of donating blood?” The study
randomly assigns individuals to three groups: in the control group there is no intervention; in the
“strong active decision group” (hereafter strong AD), individuals are asked to either agree to
participate at a specific date and time, or alternatively to refuse to participate in the blood drive
by checking a box. The third group, the “weak AD group,” is the same as the strong AD group
except that an individual is given the additional option to check a box indicating that he does not
want to make a decision now about donating blood. Under standard assumptions, where
individuals are assumed to know their preferences, the AD interventions should have no effect.
Under the constructive preferences view, the AD interventions would have a larger effect on the
subpopulation that felt they were not sufficiently informed about the importance of blood
donation.
The findings support the hypothesis that preferences in many domains do not exist for
many individuals. On the one hand, individuals who have not made up their mind about blood
donations can be nudged to donate by a subtle manipulation that makes them think about
donating blood. On the other hand, the treatment is ineffective for those who have already made
up their mind. The difference in how individuals in the two groups respond to the two treatments
is significant. Despite the subtlety of the treatment, it leads to a statistically significant effect in
the predicted subgroup of subjects. Moreover, the effect is quantitatively large—the strong AD
intervention increases the proportion of blood donors by 8 percentage points, which represents a
doubling of the sample average of 8 percent. This paper demonstrates a precise mechanism
through which default options work: Some individuals have not formed a preference, and those
individuals tend to choose the option that permits them to avoid making an active decision. If
there is no such option—that is, if individuals have to check a box to explicitly agree or
explicitly refuse to give blood—then more individuals will construct a preference. As long as
some of those who are forced to construct a preference wish to give blood, the AD treatment will
increase the number of individuals who freely show up (some days later) to donate blood. Stutzer
et al. provides new evidence against the classical economics view that individuals have fixed
preferences that are easily accessible to them.
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