Decision-making research can help economic institutions—built on the erroneous assumption that people will behave rationally—to account for predictable irrationality. It can also help us to design choice environments that lead people to make decisions that are better for them. For these reasons, Daniel Kahneman was awarded the Nobel Prize in Economics in 2002, for his contributions (with the late Amos Tversky) to the understanding of irrational decision-making.

Recent research attempts to understand where these biases come from. In most societies, humans interact with monetary markets from a young age; it seems intuitive that such exposure would be the principle source of decision-making strategies and biases. Culture and socialization must be involved, right?

But while human culture and market experience may play a role, it now seems clear that choice biases are much more deeply rooted in our biology. Previous investigations had shown that some other species—including European starlings and capuchin monkeys—may also exhibit irrational biases such as framing effects. However, because these species are fairly distant relatives of humans, it is difficult to know if framing effects are shared as a result of common ancestry, or if they evolved independently in each species. To address this question, my colleagues, Alexandra Rosati and Brian Hare, and I investigated attribute framing in human beings’ closest living relatives, bonobos and chimpanzees.

We tested 23 chimpanzees at Tchimpounga Chimpanzee Sanctuary in the Republic of Congo and 17 bonobos at Lola ya Bonobo sanctuary in the Democratic Republic of Congo. In the study, we presented the apes with choices between several peanuts and some fruit. In the positive “gain” condition, we framed the fruit option positively. We initially presented it as a single piece of fruit, but, half the time that the apes chose it, we provided them with a second piece as well. The negative “loss” condition was identical, except that in this condition we framed the fruit option negatively. Here we presented the fruit option as two pieces of fruit, but, half the time the apes chose it, we took a piece back and only provided the ape with one.

Even though in both conditions apes who chose the fruit option received identical payoffs—a 50-50 chance of getting one or two pieces of fruit—they chose the fruit option significantly more when it was framed positively than when it was framed negatively: Apes, too, make irrational economic decisions.

Because bonobos, chimpanzees, and humans all exhibit framing effects, it is unlikely that this trait evolved independently in each lineage. Instead, it appears that choice biases are evolutionarily ancient. They were probably present in the last common ancestor of bonobos, chimpanzees, and humans, which lived about six million years ago, and may even be much older. That framing effects are shared with several non-human species also suggests that these biases are deeply rooted in our biology, and can arise in the absence of experience with uniquely human monetary markets. Choice biases may have evolved in response to certain challenges in foraging ecology, or they may represent a by-product for selection on other traits, such as emotions.