The Better Angels of Our Nature: Why Vio - By Steven Pinker Page 0,379

antisocial tendencies. They also examined all the published twin and adoption studies of criminal arrests and convictions. They estimated the heritability of aggressive behavior at around 0.44, and the heritability of criminality at around 0.75 (of which 0.33 consists of additive heritability, that is, variation that breeds true, and 0.42 consists of nonadditive heritability, variation caused by interactions among genes). Though their dataset on criminality did not distinguish violent from nonviolent crimes, they cited a Danish twin study that separated the two and that yielded a heritability estimate of 0.50 for the violent ones.145 As in most studies in behavioral genetics, the effects of being brought up in a given family were tiny to nonexistent, though other aspects of the environment that are not easily measurable by these techniques, such as effects of the neighborhood, subculture, or idiosyncratic personal experiences, undoubtedly do have effects. The exact numbers should not be taken too seriously, but the fact that they are all substantially above zero should be. Behavioral genetics confirms that aggressive tendencies can be inherited, and that gives natural selection material to work with in shifting the average violent tendencies of a population.

Heritability is a necessary condition for evolutionary change, but it measures a hodgepodge of diverse contributors to behavior. When we tease them apart, we find many specific pathways by which natural selection could adjust our inclinations toward or away from violence. Let’s consider a few.

Self-domestication and pedomorphy. Richard Wrangham has noted that the domestication of animals usually tames them by slowing down components of the developmental timetable to retain juvenile traits into adulthood, a process called pedomorphy or neoteny. 146 Domesticated strains and species tend to have more childlike skulls and faces, to show fewer sex differences, to be more playful, and to be less aggressive. These changes can be seen in farm animals that have been deliberately domesticated, such as horses, cattle, goats, and foxes, and in one species of wolf that was self-domesticated after it started to hang around human campsites thousands of years ago scrounging leftover food and eventually evolved into dogs. And in chapter 2 we saw that bonobos evolved from a chimpanzee-like ancestor via a process of pedomorphy after their foraging ecology had reduced the payoff for aggression in males. Based on pedomorphic changes in the fossils of Paleolithic humans, Wrangham has suggested that a similar process has been taking place in human evolution during the past thirty to fifty thousand years, and may still be taking place.

Brain structure. The neuroscientist Paul Thompson has shown that the distribution of gray matter in the cerebral cortex, including the dorsolateral prefrontal region, is highly heritable: it is almost identical in identical twins and considerably less similar in fraternal twins.147 So is the distribution of white matter connecting the frontal cortex to other regions of the brain.148 It is possible, then, that the frontal lobe circuitry that implements self-control varies genetically among individuals, making it eligible for recent natural selection.

Oxytocin, the so-called cuddle hormone which encourages sympathy and trust, acts on receptors in several parts of the brain, and the number and distribution of those receptors can have dramatic effects on behavior. In a famous experiment, biologists inserted a gene for the receptor for vasopressin (a hormone similar to oxytocin that operates in the brains of males) into meadow voles, an aggressive and promiscuous species that lacks it. Higamous, hogamous, the voles were monogamous, just like their evolutionary cousins the prairie voles, which come with the receptors preinstalled.149 The experiment suggests that simple genetic changes in the oxytocin-vasopressin system can have profound effects on sympathy, bonding, and by extension the inhibition of aggression.

Testosterone. A person’s response to a challenge of dominance depends in part on the amount of testosterone released into the bloodstream and on the distribution of receptors for the hormone in his or her brain.150 The gene for the testosterone receptor varies across individuals, so a given concentration of testosterone can have a stronger effect on the brains of some people than others. Men with genes that code for more sensitive versions of the receptor have a greater surge of testosterone when conversing with an attractive woman (which can lead to reduced fear and greater risk-taking), and in one study were overrepresented in a sample of convicted rapists and murderers.151 The genetic pathways that regulate testosterone are complicated, but they offer a target by which natural selection could alter people’s willingness to take up aggressive challenges.

Neurotransmitters are the molecules that are released from a neuron,