Human beings behave irrationally because evolution didn’t engineer us to be general problem-solvers, but rather to be good at resolving very specific problems, Jonathan Cohen, PhD, MD, told audience members at APA’s Annual Convention.

Cohen, the Princeton University neuroscientist and cognitive psychologist who received this year’s APA Distinguished Scientific Contribution Award, said that we often don’t act in our best interests because evolution doesn’t care about the big picture. What we consider irrational usually makes sense within some other context. It’s just that “we find ourselves in circumstances to which we haven’t yet adapted,” he said

Different regions of our brain evolved at different times in response to various environmental and social pressures, Cohen said. So our brain, far from being a single cohesive unit, is made up of multiple interacting systems.

Cohen used the analogy of the United States: It’s one country composed of many parts. Each part has its own interests and often those interests compete. But with states, you can look at voting records and polls to figure out who’s doing what.

The brain is trickier — and that’s where neuroimaging comes in. “Neuroimaging allows us to crack open the box and see the engagements among these various systems,” Cohen said.

Once you’ve done that, you can begin to make some sense of humans’ seemingly irrational behavior. For example, in responding to the now-infamous “trolley problem,” people often report that they’d be willing to switch a trolley track in order to have a runaway trolley kill one person instead of five. But if you ask whether they’d push a very large person off a footbridge to stop the trolley, thereby saving five people, they’re much less likely to say yes. The end result’s the same, so why the disparity?

Part of the answer lies in what neuroimaging has shown us about decision-making. According to Cohen, one potential explanation goes like this: The brain’s limbic system, which evolved before mammals and reptiles parted ways, doesn’t care much for the future. It knows at a basic level that pushing someone off a bridge is wrong because it damages interpersonal relationships. The brain’s frontal lobe evolved much more recently and cares very much about the future. It is able to reason about the relative value of sacrificing an innocent person.

Pushing a button to switch the track doesn’t activate the limbic system’s response — after all, pushing buttons didn’t mean anything to our proto-mammalian ancestors. But it kicks in when we’re asked to physically interact with someone. Try as we like, the modern “rational” decision has to compete with our ancient responses, Cohen said.

For a more detailed look at the trolley problem and how philosophical questions are influencing psychological inquiry, read Beth Azar’s article in next month’s Monitor.