Errors of reductionism and biodeterminism take over in such silly statements as “Intelligence is 60 percent genetic and 40 percent environmental.” A 60 percent (or whatever) “heritability” for intelligence means no such thing. We shall not get this issue straight until we realize that the “interactionism” we all accept does not permit such statements as “Trait x is 29 percent environmental and 71 percent genetic.” When causative factors (more than two, by the way) interact so complexly, and throughout growth, to produce an intricate adult being, we cannot, in principle, parse that being’s behavior into quantitative percentages of remote root causes. The adult being is an emergent entity who must be understood at his own level and in his own totality. The truly salient issues are malleability and flexibility, not fallacious parsing by percentages. A trait may be 90 percent heritable, yet entirely malleable.

In fact, happiness is one of the most highly heritable aspects of personality. Twin studies generally show that from 50 percent to 80 percent of all the variance among people in their average levels of happiness can be explained by differences in their genes rather than in their life experiences. 28 (Particular episodes of joy or depression, however, must usually be understood by looking at how life events interact with a person’s emotional predisposition.)

...the results have consistently suggested that introversion and extroversion, like other major personality traits such as agreeableness and conscientiousness, are about 40 to 50 percent heritable.

The conclusion that all these studies converge upon is that about half of your IQ was inherited, and less than a fifth was due to the environment you shared with your siblings – the family. The rest came from the womb, the school and outside influences such as peer groups. But even this is misleading. Not only does your IQ change with age, but so does its heritability. As you grow up and accumulate experiences, the influence of your genes increases. What? Surely, it falls off? No: the heritability of childhood IQ is about forty-five per cent, whereas in late adolescence it rises to seventy-five per cent. As you grow up, you gradually express your own innate intelligence and leave behind the influences stamped on you by others. You select the environments that suit your innate tendencies, rather than adjusting your innate tendencies to the environments you find yourself in. This proves two vital things: that genetic influences are not frozen at conception and that environmental influences are not inexorably cumulative. Heritability does not mean immutability.

While genes are pivotal in establishing some aspects of emotionality, experience plays a central role in turning genes on and off. DNA is not the heart’s destiny; the genetic lottery may determine the cards in your deck, but experience deals the hand you can play. Scientists have proven, for example, that good mothering can override a disadvantageous temperament.

This is implied in the assertion that a man shows certain behavior because he was “born that way.” To object to this is not to argue that behavior is never determined by hereditary factors. Behavior requires a behaving organism which is the product of a genetic process. Gross differences in the behavior of different species show that the genetic constitution, whether observed in the body structure of the individual or inferred from a genetic history, is important. But the doctrine of “being born that way” has little to do with demonstrated facts. It is usually an appeal to ignorance. “Heredity,” as the layman uses the term, is a fictional explanation of the behavior attributed to it.

Schwartz's research suggests something important: we can stretch our personalities, but only up to a point. Our inborn temperaments influence us, regardless of the lives we lead. A sizeable part of who we are is ordained by our genes, by our brains, by our nervous systems. And yet the elasticity that Schwartz found in some of the high-reactive teens also suggests the converse: we have free will and can use it to shape our personalities.

Intellectual education influences the head and values-based education influences the heart.

Instead, in our world, nature's contribution to development comes not by providing a finely detailed sketch of a finished product, but by providing a complex system of self-regulating recipes. Those recipes provide for many different things-from the construction of enzymes and structural proteins to the construction of motors, transporters, receptors, and regulatory proteins-and thus there is no single, easily characterizable genetic contribution to the mind. In the ongoing, everyday functioning of the brain, genes supervise the construction of neurotransmitters, the metabolism of glucose, and the maintenance of synapses. In early development, they help to lay down a rough draft, guiding the specialization and migration of cells as well as the initial pattern of wiring. In synaptic strengthening, genes are a vital participant in a mechanism by which experience can alter the wiring of the brain (thereby influencing the way that an organism interprets and responds to the environment). There are at least as many different genetic contributions to the mind and brain as there are genes; each contributes by regulating a different process.

For the most part we inherit our opinions. We are the heirs of habits and mental customs. Our beliefs, like the fashion of our garments, depend on where we were born. We are molded and fashioned by our surroundings.

In recent years, for example, there has been much interest in the idea that one of the most fundamental factors in explaining personality is birth order: older siblings are domineering and conservative, younger siblings more creative and rebellious. When psychologists actually try to verify this claim, however, their answers sound like the Hartshorne and May conclusions. We do reflect the influences of birth order but, as the psychologist Judith Harris points out in The Nurture Assumption, only around our families. When they are away from their families — in different contexts — older siblings are no more likely to be domineering and younger siblings no more likely to be rebellious than anyone else. The birth order myth is an example of the FAE in action.

In fine, human character is moulded by a thousand subtle influences; by example and precept; by life and literature; by friends and neighbours; by the world we live in as well as by the spirits of our forefathers, whose legacy of good words and deeds we inherit. But great, unquestionably, though these influences are acknowledged to be, it is nevertheless equally clear that men must necassarily be the active agents of their own well-being and well-doing; and that, however much the wise and the good may owe to others, they themselves in the very nature of things be their own best helpers (p.31).

The adult being is an emergent entity who must be understood at his own level and in his own totality. The truly salient issues are malleability and flexibility, not fallacious parsing by percentages. A trait may be 90 percent heritable, yet entirely malleable. A twenty-dollar pair of eyeglasses from the local pharmacy may fully correct a defect of vision that is 100 percent heritable. A “60 percent” biodeterminist is not a subtle interactionist, but a determinist on the “little bit pregnant” model.

The influence upon our intelligence of events that happened in the womb is three times as great as anything our parents did to us after our birth.