the source: “The Vulcanization of the Human Brain: A Neural Perspective on Interactions Between Cognition and Emotion” by Jonathan D. Cohen, in The Journal of Economic Perspectives, Fall 2005.

How do we make decisions? Why do we allow our emotions to get in the way of rational response? What we think of as emotional behavior may be the result of “evolutionarily old” mechanisms winning out over areas of the brain that developed later in the course of human evolution, argues psychologist Jonathan D. Cohen, director of the Center for the Study of the Brain, Mind, and Behavior at Princeton University. While emotional behavior sometimes seems irrational in a modern setting, it may have been perfectly reasonable in the early days of our evolutionary history.

In this view, the human mind is best thought of not as a unified whole but rather as a “society of minds,” each capable of independent action. So although the brain’s prefrontal cortex enables the individual to act in accordance with abstract goals or prin­ciples, it doesn’t always run the show. The older, “limbic” system of the brain acts more quickly and thus may win the battle to determine behavior.

This theory resolves long-standing conundrums in various fields, such as the inconsistencies of individual mor­al behavior illustrated by the switch and footbridge scenarios.

In the switch scenario, individuals are asked if they would flip a switch to divert a trolley car onto a sidetrack if it would kill one person but save five others who are on the main track. Most people say yes.

In the footbridge scenario, they are asked if they would push a man off a footbridge onto the track below to save the same five people; in this instance, most people say no. We instinctively recoil from the idea of pushing someone off a bridge, but if we can flip a switch from a distance, we seem able to make the rational choice.

What explains the difference? In his work using magnetic resonance imaging (MRI) to monitor brain activity, Cohen sees an answer in the “society of minds” theory.

In people faced with dilemmas like  the footbridge scenario, MRIs revealed activity in the emotional processing regions of the brain, such as the medial prefrontal cortex. The switch scenario, however, triggered activity in the anterior and dorsolateral areas of the prefrontal cortex, home of more-rational thought processes.

Cohen is careful to note that MRIs, which measure changes in blood oxygen in specific areas of the brain, are not a decisive indicator of brain activity. And even a correlation between brain activity and behavior does not prove that one caused the other.

Why would people have developed a negative emotional response to pushing someone off a bridge? One possibility is that an aversion to killing arose because it fostered the creation of cooperative social structures that conferred an evolutionary advantage.

Many seemingly irrational human decisions observed by behavioral economists can also be explained by the domin­ance of evolutionarily old emotional responses. In the ulti­matum game, for example, a player is given a sum of money and instructed to make an offer to a partner about how it should be split between them. If they can’t agree on a split, both players get nothing. Surprisingly, people in tests run in many different cultures generally reject offers of less than 20 percent of the sum, often walking away empty handed.

This, too, seems to be a deeply embedded response—Cohen suggests that early humans living in small groups needed to show their fellows that they couldn’t be taken advantage of—and it’s associated with activity in more primitive areas of the brain. The contemporary human preference for immediate consumption (think failure to save) also falls into this cate­gory; the best place for our evolutionary ancestors to store food was in their bellies.

It’s the rational mind that has created today’s complex technological societies, Cohen observes, but the often discordant “society of minds” in our heads isn’t always up to the challenges those modern societies pose.