Happiness is one of the interesting traits that I call "emergenic." Although they have strong genetic roots, hence the strong MZ correlations, the negligible similarity of DZ twins indicates that these traits do not tend to run in families. Metrical traits that do run in families, traits like stature, reflect the additive combination of polygenic effects(the lengths of the head, neck, torso, upper and lower leg add up to body height. Emergenic traits seem to involve configural rather than additive combinations of the polygene effects, so that small gene changes can produce large changes in the trait. Because each parent contributes just half of her or his genes to each child, and because siblings share on average just half of their polymorphic genes, first-degree relatives are unlikely to share all of the genes involved in an emergenic configuration.

Facial beauty seems to be an emergenic trait as is the distinctive quality of the singing or speaking voice. MZ twins can usually fool even family members by impersonating their cotwins on the telephone; DZ twins very seldom can do this. Music majors at my university, including those specializing in voice, commonly have musical parents, but the voice major seldom have parents who sing. The racing ability of the legendary stallion, Secretariat, seems to have been emergenic. Mated with only the most promising mares, he produced more than 400 foals, only one of them(Risen Star) was a winner and even he could not have run with dad.

Were it not for ideological prejudice, any rational person looking at the evidence would agree that human aptitudes, personality traits, many interests and personal idiosyncrasies, even some social attitudes, owe from 30 to 70 percent of their variation across people to the genetic differences between people. The ideological barrier seems to involve the conviction that accepting these facts means accepting biological determinism, Social Darwinism, racism, and other evils. I myself fall prey to this mistake from time to time. In the paper reporting our happiness data, for example, noting that the happiness set-point is largely genetic while the events that move us temporarily above or below our set-points are largely fortuitous, I wrote: "perhaps trying to be happier is like trying to be taller." To make up for this error, I have had to write a book (Happiness: Its Nature and Nurture) explaining why trying to be happier is both feasible and fun.

The actual mechanism by which the genes affect the mind is still what Pinker (and Noam Chomsky) would call a mystery rather than a mere problem. We do not have a clue about how the brain module that permits humans but not chimps to acquire language is actually fashioned by the genetic enzyme factory. In the comparatively simple brain of a chicken there is a gizmo that produces an alarm reaction when the silhouette of a flying hawk passes overhead, but not when it is passed backwards so that it looks more like a flying chicken. We cannot locate that gizmo or describe its construction. We do not know which genes in the chicken DNA mutated eons ago to bring about this adaptive response and we clearly have no idea at all as to how these genes manage to fabricate this gizmo in every modern chicken's brain. Yet the existence of the gizmo cannot be doubted.

In the case of most human psychological traits, however, an important part of the mechanism is less mysterious. We know that, to an important extent, the genes affect the human mind indirectly by influencing the kinds of experiences we have, the way in which other people react to us, and especially by influencing the kinds of environments we seek out and the ways in which we react to our experiences. For genetic reasons, some babies are fretful and unresponsive while others tend to smile and coo. These different behaviors elicit different parenting responses. A genetically venturesome toddler climbs on things, falls off, explores, knocks things over, and has physical and social experiences that his more sedentary sibling seldom has. A naturally bright, inquisitive youngster notices and thinks about things, reads more, asks more questions, and elicits better answers than does a child whose mental processes are slower and less intrinsically rewarding. A little boy who is at the low end of the normal distribution of genetic fearfulness is less easily intimidated by the punishment on which both parents and peers tend to rely in trying to modify that boy's behavior. Many parents of such children give up the battle and the child remains unsocialized, a kind of psychopath. More skillful and persistent parents emphasize reward instead of punishment, work to instill pride rather than guilt. A fearless child left to himself is likely to become a leader of the gang, delinquent, then criminal, but with skillful parenting that same boy can grow to be the kind of man we like to have around when danger threatens. I think that the hero and the psychopath are twigs on the same genetic branch.

Thus, genetic effects on human psychology are often distal in the causal chain while the proximal causes are environmental, just as those reactionary Lockians have always claimed. A better formula than Nature versus Nurture would be Nature via Nurture. But, distal or not, the genetic influences are strong and most of us develop along a path determined mainly by our personal genetic steersmen. It is often possible to intervene but it is seldom easy. A genetically timid child, for example, can be desensitized by carefully calibrated exposures to increasingly stressful situations. Meanwhile, a genetically venturesome child, a boy like General Chuck Yeager for example, is doing the same thing much faster on his own, climbing higher, taking greater risks, learning to fly, becoming a fighter pilot, then an ace, then a test pilot, and finally breaking the sound barrier.