Genius

The first link in the chain is inheritance, and Eysenck, a lifelong student of the genetics of personality, is comfortable there. Twin and family studies had by then established that traits like intelligence and temperament are substantially heritable — not fixed by genes, but strongly shaped by them. What he adds is a candidate trait, one he had spent years developing on his own personality scales: psychoticism. The name is unfortunate and he knew it. It does not mean psychosis. It marks a dimension along which everyone sits, describing a cluster of tendencies — a taste for the unconventional, a certain coldness or detachment, aggression, impulsivity, a tolerance for the odd that most people quietly avoid.

People high on this dimension are not ill. But they sit closer, on a shared continuum, to those who are — and closer, too, to the relatives of the seriously disturbed, who often show unusual creativity without the breakdown. Eysenck's claim is that this dimension, largely inherited, is the missing ingredient Terman's well-adjusted prodigies lacked. They had the intelligence. What they didn't have was the temperament that lets a mind wander somewhere disreputable and stay there.

The cognitive link Eysenck seizes on is an old clinical observation: overinclusive thinking. It had first been described in patients with schizophrenia, whose thought seemed unable to keep the fence around a concept — irrelevant associations flooding in, boundaries blurring, categories bleeding into one another. In the clinic this is a symptom, a failure. Eysenck's insight is that the very same loosening, in a person with the intelligence to steer it, is not a failure at all. It is the engine of original combination.

Creativity, on this account, is at bottom the pairing of ideas that don't ordinarily belong together — a remote analogy, an image dragged in from a distant field, a connection nobody else's mind would offer up because everyone else's mind keeps its categories tidy. The overinclusive thinker has access to a much wider net of associations. Most of what that net catches is junk; a mind flooded with loose connections is, more often than not, just noisy. But occasionally it lands the improbable link that turns out to be the breakthrough, the one a well-fenced mind would never reach.

Step back from the individual links and what Eysenck has really attempted comes into view. He has taken the most romantic subject in the culture — the lone titan touched by something we can't name — and refused it the exemption we usually grant it. Genius, in his hands, is not a gift from the gods or an accident of grace. It is the visible top of a chain that can, in principle, be measured at every level: heritability at the bottom, a personality trait in the middle, a cognitive style above it, achievement at the top. Each link is something you can put a number on, or at least argue about with evidence.

That is the ambition, and it is worth naming plainly because it cuts against how we prefer to talk about greatness. We like our geniuses inexplicable; the mystery flatters both them and us. Eysenck's whole project is a refusal of that comfort. If the theory is right, the melancholy Beethoven and the impossible Newton are not exceptions to psychology but cases of it — extreme values on dimensions the rest of us occupy at gentler settings. The distance between us and them becomes a difference of degree, sitting on the same continuum, rather than a difference of kind.

The book ends more or less where the intuition began, but the intuition has been rebuilt. That old suspicion — that the greatest minds are a little touched — arrives at the last page no longer as gossip but as the top of a structure. Genes tilt a temperament; the temperament loosens the fences around thought; the loosened thought, in a mind clever and stubborn enough to sift it, throws up the connection that changes a field. The strangeness the biographies keep reporting turns out to be load-bearing.