Humans are generally reluctant to give up on something they’ve already committed time and effort to. It’s called the ‘sunk costs’ phenomenon, where the more resources we sink into an endeavor, the likelier we are to continue — even if we sense it’s futile. A new study shows that both capuchin monkeys and rhesus macaques are susceptible to the same behavior and that it occurs more often when the monkeys are uncertain about the outcome.

But why would we engage in such potentially self-defeating behavior?

Georgia State researchers think two factors may play a role. First, it may be a deep, evolutionarily ancient mechanism that helps us balance overall cost and benefit. Second, it may be influenced by uncertainty about the outcome (you never know, it might work out, so why not keep trying?)

Julia Watzek, a recent Georgia State University Ph.D. recipient, and her graduate advisor Professor Sarah F. Brosnan have shown that both capuchin monkeys and rhesus macaques are susceptible to the same behavior and that it occurs more often when the monkeys are uncertain about the outcome. Their new study, «Capuchin and rhesus monkeys show sunk cost effects in a psychomotor task,» was published recently in Nature’s Scientific Reports.

The monkeys are housed at the university’s Language Research Center, where they have indoor and outdoor areas to live and play in, and participate in entirely voluntary and non-invasive cognitive and behavioral research, said Brosnan, who is affiliated with the Department of Psychology and the Center for Behavioral Neuroscience. In fact, she has worked with some of these monkeys for over twenty years. «They’re like my second set of kids,» she said.

In the study, 26 capuchin monkeys and 7 rhesus macaques got to play a simple video game where they operated a joystick, and they needed to move a cursor onto a moving target and keep it there while the target kept moving. If they were successful, they heard a «whoop» sound that indicated success and got a treat. If their cursor lost contact with the moving target, they didn’t get a reward and a new round began. After being trained, the experiment tested them on rounds of either 1, 3 or 7 seconds. «Monkeys have really quick reaction times on these games,» said Brosnan, «so one second to them is actually a long time.»

In fact, Watzek said, «Most rounds lasted only 1 second. So if you didn’t get a reward after that, it was actually better to quit and start a new round. That would likely get you a treat sooner than if you had kept going.»

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Materials provided by Georgia State University. Note: Content may be edited for style and length.