Apes Prefer the Glass Half Full

Nearest primate relatives also susceptible to marketing spin

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Kiki is a chimpanzee at Tchimpounga Chimpanzee Sanctuary in the Republic of Congo. A new study of chimpanzees and bonobos finds that humans aren’t the only species susceptible to “glass half empty" thinking. Photo by Alexandra Rosati, Duke University.

Humans aren’t the only species to be influenced by spin. Our closest primate relatives are susceptible, too.

For example, people are known to rate a burger as more tasty when it is described as “75 percent lean” than when it is described as “25 percent fat," even though that's the same thing. And they're more willing to recommend a medical procedure when they are told it has a 50 percent success rate than when they are told it has a 50 percent chance of failure -- again, exactly the same thing.

A Duke University study has found that positive and negative framing make a big difference for chimpanzees and bonobos too.

In experiments conducted at Tchimpounga Chimpanzee Sanctuary in the Republic of Congo and Lola ya Bonobo Sanctuary in the Democratic Republic of Congo, researchers presented 23 chimpanzees and 17 bonobos with a choice between two snacks -- a handful of nuts and some fruit.

In one series of trials, the researchers framed the fruit option positively -- by offering one piece of fruit, with a 50 percent chance of a surprise bonus piece.

In another series of trials, the researchers framed the fruit option negatively. This time they offered two pieces of fruit rather than one, but if the apes chose the fruit, half the time they were shortchanged and received only one piece instead.

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Yolo is a bonobo at Lola ya Bonobo Sanctuary in the Democratic Republic of Congo

Chimps and bonobos were more likely to choose the fruit over the nuts when they were offered a smaller amount of fruit but sometimes got more, versus when they were initially offered more but sometimes got less -- despite receiving equal average payoffs in both scenarios.

The preference for the option framed as a prize rather than a penalty was especially strong in males, the researchers found.

Scheduled to appear Feb. 11 in the journal Biology Letters, the study is part of a larger body of research on how psychological factors shape behavior and decision-making.

“People tend to prefer something more when you accentuate its positive attributes than when you highlight its negative attributes, even when the options are equal,” said Christopher Krupenye, a doctoral student in evolutionary anthropology at Duke who co-authored the study with Duke researcher Brian Hare and Alexandra Rosati of Yale.

“Historically, researchers thought these kinds of biases must be a product of human culture, or the way we’re socialized, or our experience with financial markets. But the fact that chimps and bonobos, our closest living primate relatives, exhibit the same biases suggests they’re deeply rooted in our biology,” Krupenye said.

“That means it’s very difficult to overcome these biases, but it is possible to create environments that might help us make better choices.”

This research was supported by grants from the National Science Foundation (DGE-1106401, BCS-08– 27552–02, BCS-10–25172), the L.S.B. Leakey Foundation and by European Research Commission Advanced Grant Agreement 233297.