Once invested, Monkeys, like Humans, Keep on Going

If you’ve ever stayed in a relationship too long or stuck with a project that was going nowhere, you’re not alone.  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.

Study overview

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, so one second to them is actually a long time.  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.

Study findings overview

The researchers found that both species of monkeys showed sunk cost effects. They persisted 5 to 7 times longer than was optimal and the longer they had already tried, the more likely they were to complete the entire task.  Uncertainty played a large part, because when the monkeys got a signal that additional work was required, they were less susceptible to sunk cost behavior, though they still did demonstrate it.

So why do monkeys (and humans) keep on keepin’ on, long after it looks fruitless?

The researchers say the results suggests that this behavior is likely driven by evolution and deeply embedded across species.

“The epitome of the sunk cost is I’ve invested so much in this, I’m just going to keep going. And there may be benefits to this. Sometimes, you need to have patience. That helps when you’re foraging for food, hunting prey, waiting for eggs to hatch, seeking a mate, or building a nest or enclosure.”

-Professor Sarah F. Brosnan

Journal Reference:  Julia Watzek, Sarah F. Brosnan. Capuchin and rhesus monkeys show sunk cost effects in a psychomotor task. Scientific Reports, 2020; 10 (1) DOI: 10.1038/s41598-020-77301-w