Sad Brain, Happy Brain

The fear system is extraordinarily efficient. It is so efficient that you don't need to consciously register what is happening for the brain to kick off a response. If a car swerves into your lane of traffic, you will feel the fear before you understand it. Signals travel between the amygdala and your crisis system before the visual part of your brain has a chance to "see." Organisms with slower responses probably did not get the opportunity to pass their genetic material along.

Fear is contagious because the amygdala helps people not only recognize fear in the faces of others, but also to automatically scan for it. People or animals with damage to the amygdala lose these skills. Not only is the world more dangerous for them, the texture of life is ironed out; the world seems less compelling to them because their "excitement" anatomy is impaired.

Anger Management
Until recently, there was relatively little research showing how the brain processes anger. But that has begun to change. Recent studies indicate that anger may trigger activity in a part of the brain not named as poetically as the amygdala—the dorsal anterior cingulate cortex (abbreviated dACC). Like the amygdala, the dACC's function makes sense, given its connections to areas of the brain involved in recognizing an offense (he just stole my iPod), registering a feeling (I'm angry) and acting on it (I'm going to …). It also links to the reasoning centers in the front part of the brain, as well as memory centers, which play a role in angry rumination or stewing after the fact.

Researchers, however, have been more focused on one of the consequences of anger—aggression—probably because it can be observed through behavior. It's known, for example, that men are overtly more aggressive than women because of differences in male and female hormones. But the brains of men and women are also different, and some of those differences may affect aggression. In the front of the brain, the orbitofrontal cortex is recruited to help make decisions and temper emotional responses. It lights up when people are making judgments. Adrian Raine and colleagues at the University of Southern California note that, on average, men have a lower volume of gray matter (the bodies of nerve cells) in the orbitofrontal cortex than women. According to their analysis, this brain difference accounts for a healthy portion of the gender gap seen in the frequency of antisocial behavior.

Even a neuroscientist can see that murder and mayhem are undesirable. But a neuroscientist can also see why that trait might still be in the gene pool. The gene for sickle cell anemia survived because it provided protection against another disease, malaria. Similarly, aggression is often an advantage. Until recently in historical terms, a readiness to fight and the ability to kill was a way to consolidate control over resources for survival.

Fortunately, diplomats have also evolved. Some of our ancestors who understood that aggression carried risks as well as advantages used their creative human brains to devise better solutions for resolving conflicts. Our predecessors also originated symbolic diversions for aggression, like sports and chess.