International Journal of Comparative Psychology

The endowment effect is the tendency to, seemingly irrationally, immediately value a possessed item more than the opportunity to acquire the identical item when one does not already possess it. Although endowment effects are reported in chimpanzees (Brosnan, Jones, Lambeth, Mareno,Richardson, & Shapiro, 2007) and capuchin monkeys (Lakshminarayanan, Chen, & Santos, 2008), both species share social traits with humans that make convergence as likely an evolutionary mechanism as homology. Orangutans (Pongo spp.) provide a unique insight into the evolution of the endowment effect, along with other apparently irrational behaviors, because their less frequent socialinteractions and relatively more solitary social organization distinguishes them from the more gregarious apes, allowing a test of evolutionary homology. In the present study, we used pairs of both food and non-food objects, as in an earlier test on chimpanzees (Brosnan et al., 2007). We established the apes’ preferences in forced -choice tasks, then tested whether they showed an endowment effect inan exchange task, in which subjects were given one of the objects, followed by the option to exchange it for the other. Here, we report the first evidence of the endowment effect in a relatively less social primate, the orangutan. This indicates that this behavior may have evolved as a homology within the primates, rather than being due to convergent social pressures. These findings provide stronger evidence for the hypothesis that at least one bias, the endowment effect, may be common in primates and, potentially, other species.