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The New Science of Human Individuality
By David J. Linden
In the longstanding debate over whether “nature” or “nurture” determines how we turn out, the old saw goes like this: When your first baby is born, you are sure that what matters is nurture. When your second baby is born, you’re a firm believer in nature.
This adage is confirmed, somewhat, in how people answered a survey that David J. Linden cites in “Unique: The New Science of Human Individuality.” In the book Linden, a neuroscientist at Johns Hopkins University, sets out to look at everything that makes us distinctly ourselves: our height and weight, food preferences, personality styles, gender identity, racial identity, sexual orientation and intelligence. Are these qualities carried in our genes, or does the life we live — every experience, from the viruses we encounter to the books we read to what month we were born — play a bigger role in making us who we are?
Turns out we can be bad at guessing to what degree our traits are genetic — or what scientists call “heritable” — and to what degree they’re affected by environment. In an online survey from 2019, Linden tells us, Americans were generally good at intuiting the source of certain traits; they knew that height is strongly heritable, political beliefs are not heritable at all and musical talent is somewhere in between. But they also had some revealing blind spots. They assumed that the heritability of body mass index was 40 percent, for instance, when the consensus from the scientific literature is that it’s really more like 65 to 75 percent. “I imagine that many people want to believe that food consumption is more a matter of personal willpower than it really is,” Linden writes; they want to believe “that they (and others) have a greater degree of autonomy and personal agency than they really do.”
And the survey respondents who were most accurate? College-educated mothers of more than one child. In other words, the ones whose “it’s all nurture” attitude was likely attenuated by the birth of a second child with a different “nature.”
This is not to say that Linden is out to prove that genes define us; far from it. The main takeaway from “Unique” is that while there might be a genetic tendency to develop in a particular way, there’s a wide range of influences, beginning in fetal life, that help determine how and whether our genes are expressed. Some examples of the gene-environment interplay are well known, such as the single gene that causes a cognitive impairment known as PKU, which never reveals itself if a child eats the right diet. But most of the scientific understanding is still evolving as to just how experience interacts with and changes gene expression.
I feel the need to pause here to point out that Linden hates the phrase I’ve been using: “nature versus nurture.” He says it oversimplifies the question of how genetics and the environment influence each other over the course of development.
In ordinary English, “nurture” means how your parents raise you, he writes. “But, of course, that’s only one small part of the nonhereditary determination of traits.” He much prefers the word “experience,” which encompasses a broad range of factors, beginning in the womb and carrying through every memory, every meal, every scent, every romantic encounter, every illness from before birth to the moment of death. He admits that the phrase he prefers to “nature versus nurture” doesn’t roll as “trippingly off the tongue,” but he offers it as a better summary of how our individuality really emerges: through “heredity interacting with experience, filtered through the inherent randomness of development.”
There’s a lot of interesting stuff in “Unique,” including findings from decades’ worth of twin studies that gave scientists some of their first insights into how much of personality and behavior might be inherited and how much acquired. There are some great descriptions of investigations into genetics including, for instance, a Russian geneticist’s attempt, starting in the 1950s, to domesticate silver foxes by breeding them for tameness. Or we might find ourselves in the middle of a cool psych experiment, like one conducted at Berkeley in 2006 that ended up with a bunch of blindfolded college kids crawling through a grassy field trying to smell a trail of chocolate.
More notably, Linden marches into territory where too many other scientists fear to tread: the genetics of gender identity, sexual orientation and race. He manages, by and large, to avoid the worst land mines, acknowledging certain genetic differences among some populations but emphasizing that they don’t necessarily align with the social binaries of male/female, gay/straight, cis/trans or Black/white. Occasionally I did find myself cringing at some of the language, as in the chapter on gender, where I wish he had followed his own good advice — offered in a footnote in which he referred readers to the advocacy group InterACT — and steered clear of medicalized language to describe intersex people. But such missteps were rare.
I also wish he had worked harder to explain some of the complicated biological processes at the heart of his argument, especially epigenetics and neurogenetics. Too often he resorts to the crutch of apologizing in advance for prose riddled with “alphabet soup,” or for “bombarding you with a bunch of names for biomolecules,” before launching into a tangle of jargon. But apologizing in advance shouldn’t get him off the hook. It’s possible to convey even complicated biology in crisp, clear language without sacrificing accuracy. It’s just hard — especially for a scientist like Linden who clearly knows his subject inside out — and Lord knows it’s time-consuming. But it’s worth the effort.
Still, when it matters, Linden neither falters nor apologizes. At the end of his chapter on the genetics of race, he could not be clearer. “I can’t say this loudly enough,” he writes, making liberal use of italics: “There is no evidence for significant average differences in intelligence-related genes between ‘races.’ Not between self-identified whites and Blacks in the United States, nor between any pair of self-defined racial groups. Not only that, there is no evidence for racial group differences in genes that have been linked to any behavioral or cognitive trait. Not aggression. Not A.D.H.D. Not extroversion. Not depression. Nada, niente, nichts, bupkis.”
That’s the kind of clarity we need more of in popular science books like this, especially ones that investigate both what makes us human and what makes us distinctly, immutably ourselves.