The Gene: An Intimate History - Siddhartha Mukherjee Page 0,177

are part of the human genome, and as geneticists discover more powerful methods to map, identify, and categorize genes, we will inevitably find some of these determinants. Like gender, these elements will likely be hierarchically organized—with master regulators on top, and complex integrators and modifiers on the bottom. Unlike gender, though, sexual identity is unlikely to be governed by a single master regulator. Multiple genes with small effects—in particular, genes that modulate and integrate inputs from the environment—are much more likely to be involved in the determination of sexual identity. There will be no SRY gene for straightness.

The publication of Hamer’s article on the gay gene coincided with the forceful reemergence of the notion that genes could influence diverse behaviors, impulses, personalities, desires, and temperaments—an idea that had been out of intellectual fashion for nearly two decades. In 1971, in a book titled Genes, Dreams and Realities, Macfarlane Burnet, the renowned Anglo-Australian biologist, wrote, “It is self-evident that the genes that we are born with provide, along with the rest of our functional selves, the basis of our intelligence, temperament and personality.” But by the midseventies, Burnet’s conception had become far from “self-evident.” The notion that genes, of all things, could predispose humans to acquiring particular “functional selves”—possessing particular variants of temperament, personality, and identity—had unceremoniously been drummed out of universities. “An environmentalist view . . . dominated psychological theory and research from the 1930s through the 1970s,” Nancy Segal, the psychologist, wrote. “Other than being born with a general capacity to learn, human behavior was explained almost exclusively by forces outside the individual.” A “toddler,” as one biologist recalled, was perceived as a “random access memory onto which any number of operating systems could be downloaded by culture.” The Silly Putty of a child’s psyche was infinitely malleable; you could mold it into any shape and force it into any dress by changing the environment or reprogramming behavior (hence the stupefying credulity that enabled experiments, such as John Money’s, to attempt definitive changes in gender using behavioral and cultural therapy). Another psychologist, entering a research program at Yale University in the 1970s to study human behaviors, was bewildered by the dogmatic stance against genetics in his new department: “Whatever back-porch wisdom we might have brought to New Haven about inherited traits [driving and influencing human behaviors] was the kind of bunk that we were paying Yale to purge.” The environment was all about environments.

The return of the native—the emergence of the gene as a major driver for psychological impulses—was not as easy to orchestrate. In part, it required a fundamental reinvention of that classic workhorse of human genetics: the much maligned, much misunderstood twin study. Twin studies had been around since the Nazis—recall Mengele’s macabre preoccupation with Zwillinge—but they had reached a conceptual gridlock. The problem with studying identical twins from the same family, geneticists knew, was the impossibility of unbraiding the twisted strands of nature and nurture. Reared in the same home, by the same parents, often schooled in the same classrooms by the same teachers, dressed, fed, and nurtured identically, these twins offered no self-evident way to separate the effects of genes versus the environment.

Comparing identical twins to fraternal twins partially solved the problem, since fraternal twins share the same environment, but share only half the genes, on average. But critics argued that such identical/fraternal comparisons were also intrinsically flawed. Perhaps identical twins are treated more similarly than fraternal twins by their parents. Identical twins, for instance, were known to have more similar patterns of nutrition and growth compared to fraternal twins—but was this nature or nurture? Or identical twins might react against each other to distinguish themselves from one another—my mother and her twin often self-consciously chose opposite shades of lipstick—but was that dissimilarity encoded by genes, or a reaction to genes?

In 1979, a scientist in Minnesota found a way out of the impasse. One evening in February, Thomas Bouchard, a behavioral psychologist, found a news article left by a student in Bouchard’s mailbox. It was an unusual story: a pair of identical twins from Ohio had been separated at birth, adopted by different families, and experienced a remarkable reunion at age thirty. These brothers were obviously part of a fleetingly rare group—identical twins given up for adoption and reared apart—but they represented a powerful way to interrogate the effects of human genes. Genes had to be identical in these twins, but environments were often radically different. By comparing separated-at-birth twins against twins brought up in