In Aesop’s fable of the ant and the grasshopper, a hardworking ant spends its summer days storing food for the winter ahead, while the indolent grasshopper lazes in the sunshine. A Pitt professor and his colleagues have found evidence that supports what many of us know from experience: When we face a choice involving the future, our inner ant and inner grasshopper begin to tussle.

“Human decision-makers seem to be torn between an impulse to act like the indulgent grasshopper and an awareness that the patient ant often gets ahead in the long run,” the researchers write in a paper published Oct. 15 in the journal Science.

Using brain imaging, Jonathan D. Cohen, professor of psychiatry at Pitt and director of Princeton University’s Center for the Study of Brain, Mind, and Behavior, and his coauthors found that different parts of the brain are at work when people consider instant gratification versus a delayed reward.

The study also concluded that people behave differently when rewards are offered to them at different intervals. For example, if they are offered a choice between $10 today and $11 tomorrow, most people will pick the immediate reward. But if they are offered the choice of $10 a year from now or $11 a year and a day from now, they’ll pick the higher amount and wait the extra day. In classic economic theory, this doesn’t make any sense—shouldn’t a day’s wait be a day’s wait?

Cohen and his coauthors—George Loewenstein, professor of economics and psychology at Carnegie Mellon University; Samuel McClure, postdoctoral fellow in psychology at Princeton; and David Laibson, professor of economics at Harvard University—think they have found the answer. They argue that people’s brains are engaged in an internal war, with the emotional “I want it NOW!” impulse struggling against the more logical response: “But if I wait, I’ll have more money.” Apparently, when there is only a short time span between posing the question and offering the reward, the emotional side wins out, whereas if there is a longer delay, the logical part of the brain kicks in.

“This is part of a series of studies we’ve done that illustrate that we are rarely of one mind,” said Cohen. “We have different neural systems that evolved to solve different types of problems, and our behavior is dictated by the competition or cooperation between them.”

The researchers gave problems like the one above to 14 Princeton students, then used functional magnetic resonance imaging to see which parts of the students’ brains were active. Students were offered choices between getting Amazon.com gift certificates immediately, and larger amounts they could get only by waiting for a period.

Researchers found that all the decisions the students made activated the fronto-parietal cortex, a part of the brain associated with abstract reasoning, but only those that involved the possibility of an immediate reward activated the dopaminergic/limbic system, associated with emotion.

Also, when the students had the option of being rewarded immediately, but chose to wait, their brains’ calculating regions were more strongly activated—whereas, when they chose the immediate reward, the activity of both regions was comparable, with a slight trend toward more emotional activity.

Other research supports the researchers’ conclusion. Even advanced primates, which have much smaller prefrontal cortexes than humans, have not been observed to delay gratification for more than a few minutes. And patients with prefrontal damage often engage in more impulsive behavior and lose their ability to plan.

Cohen said his group’s findings may also cast light on impulsive behavior—for example, drug abuse. “Abusing drugs taps right into the part of the brain that says, ‘I want this, I want this,’ so addicts wind up making bad choices weighing the desire for the drug against the consequences of the drug,” said Cohen.

The researchers will next investigate what kinds of rewards—and how short a delay—are needed to trigger the emotional, rather than the rational, reaction.

Cohen and his coauthors conclude that, when it comes to making decisions, “the idiosyncrasies of human preferences seem to reflect a competition between the impetuous limbic grasshopper and the provident prefrontal ant within each of us.”