This is not some namby-pamby, middle-of-the-road compromise. This is a new understanding of the fundamental building blocks of life based on the discovery that genes are not immutable things handed down from our parents like Moses' stone tablets but are active participants in our lives, designed to take their cues from everything that happens to us from the moment of our conception.

For the time being, this new awareness has taken its strongest hold among scientists, changing how they think about everything from the way bodies develop in the womb to how new species emerge to the inevitability of homosexuality in some people. (More on all this later.) But eventually, as the general population becomes more attuned to this interdependent view, changes may well occur in areas as diverse as education, medicine, law and religion. Dieters may learn precisely which combination of fats, carbohydrates and proteins has the greatest effect on their individual waistlines. Theologians may develop a whole new theory of free will based on the observation that learning expands our capacity to choose our own path. As was true of Copernicus's observation 500 years ago that the earth orbits the sun, there is no telling how far the repercussions of this new scientific paradigm may extend.

To appreciate what has happened, you will have to abandon cherished notions and open your mind. You will have to enter a world in which your genes are not puppet masters pulling the strings of your behavior but puppets at the mercy of your behavior, in which instinct is not the opposite of learning, environmental influences are often less reversible than genetic ones, and nature is designed for nurture.

Fear of snakes, for instance, is the most common human phobia, and it makes good evolutionary sense for it to be instinctive. Learning to fear snakes the hard way would be dangerous. Yet experiments with monkeys reveal that their fear of snakes (and probably ours) must still be acquired by watching another individual react with fear to a snake. It turns out that it is easy to teach monkeys to fear snakes but very difficult to teach them to fear flowers. What we inherit is not a fear of snakes but a predisposition to learn a fear of snakes—a nature for a certain kind of nurture.

Before we dive into some of the other scientific discoveries that have so thoroughly transformed the debate, it helps to understand how deeply entrenched in our intellectual history the false dichotomy of nature vs. nurture became. Whether human nature is born or made is an ancient conundrum discussed by Plato and Aristotle. Empiricist philosophers such as John Locke and David Hume argued that the human mind was formed by experience; nativists like Jean-Jacques Rousseau and Immanuel Kant held that there was such a thing as immutable human nature.

It was Charles Darwin's eccentric mathematician cousin Francis Galton who in 1874 ignited the nature-nurture controversy in its present form and coined the very phrase (borrowing the alliteration from Shakespeare, who had lifted it from an Elizabethan schoolmaster named Richard Mulcaster). Galton asserted that human personalities were born, not made by experience. At the same time, the philosopher William James argued that human beings have more instincts than animals, not fewer.

In the first decades of the 20th century, nature held sway over nurture in most fields. In the wake of World War I, however, three men recaptured the social sciences for nurture: John B. Watson, who set out to show how the conditioned reflex, discovered by Ivan Pavlov, could explain human learning; Sigmund Freud, who sought to explain the influence of parents and early experiences on young minds; and Franz Boas, who argued that the origin of ethnic differences lay with history, experience and circumstance, not physiology and psychology.

Galton's insistence on innate explanations of human abilities had led him to espouse eugenics, a term he coined. Eugenics was enthusiastically adopted by the Nazis to justify their campaign of mass murder against the disabled and the Jews. Tainted by this association, the idea of innate behavior was in full retreat for most of the middle years of the century. In 1958, however, two men began the counterattack on behalf of nature. Noam Chomsky, in his review of a book by the behaviorist B.F. Skinner, argued that it was impossible to learn human language by trial and error alone; human beings must come already equipped with an innate grammatical skill. Harry Harlow did a simple experiment that showed that a baby monkey prefers a soft, cloth model of a mother to a hard, wire-frame mother, even if the wire-frame mother provides it with all its milk; some preferences are innate.

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