Blueprint

The workhorse of the field is the twin study, and its logic depends on a small natural experiment that biology runs constantly. Identical twins come from one fertilized egg and share essentially the same DNA. Fraternal twins come from two eggs and are, genetically, ordinary siblings who happened to share a womb. Both kinds of pairs typically grow up in the same home, with the same parents and roughly the same circumstances. So if identical twins resemble each other more than fraternal twins do on some trait, the extra resemblance points to the extra DNA they share.

Plomin helped run the studies that made these comparisons at scale. The Twins Early Development Study, which he founded, has followed thousands of British twin pairs born in the mid-1990s for decades, tracking their abilities, behavior and school performance year after year. The samples are large enough that the estimates stop being anecdotes and become stable numbers. And the more striking cases — identical twins separated in infancy and reunited as adults — showed similarities in temperament, tastes and even life outcomes that no shared upbringing could explain, because there had been no shared upbringing.

For all their power, twin studies have a built-in limitation: they can tell you that a trait is heritable without pointing to a single gene responsible. For decades that gap fed a hope that specific genes for intelligence or depression would eventually be found, one gene per trait, the way a single mutation causes cystic fibrosis. That hope collapsed. When researchers finally scanned whole genomes across huge samples, they found that psychological traits are not driven by a few powerful genes but by thousands of tiny ones, each nudging the outcome by a vanishingly small amount.

This is the shift Plomin treats as the real revolution in his field. The technique is the genome-wide association study, which compares the DNA of very large groups of people to find which of the millions of common variants track a given trait. No single variant means much. But you can add them all up, weighted by their effects, into one number for one person. That number is a polygenic score, and it is the first tool in the history of behavioral genetics that measures inherited propensity directly, from an individual's DNA, rather than inferring it from how relatives resemble each other.

What Blueprint ultimately puts on the table is a new form of self-knowledge, and it arrives without instructions. A polygenic score can now be calculated for a newborn, offering a probabilistic sketch of dispositions toward reading difficulty, or anxiety, or how far the child may go in school. Nothing like this has existed before. Astrology and character-reading always traded on the appeal of a life foretold; here is a version with actual predictive validity, however partial, derived from the child's own molecules. Plomin's argument forces the question of what one is supposed to do with a number like that.

The honest answer, in his account, is that the score changes what we can foresee far more than what we can control. Heritability is not a lever. Knowing that a disposition is substantially genetic does not tell a parent or a teacher how to alter it, and in many cases the disposition may not be alterable in the way we imagine. This is the uneasy heart of the book: a science that predicts increasingly well while prescribing very little. It hands us a forecast and stays silent on the response.

Plomin spent nearly fifty years moving a single claim from the fringe to the mainstream: that the psychological differences between people are, to a large degree, written in DNA. The twin studies established that inheritance mattered more than a century of blank-slate thinking allowed. The polygenic score turned that population fact into something you can measure in one person, from a cheek swab, before a life has begun. What began as an argument about twins raised apart ends as a number that can be printed at birth.