What Makes You Unique?

A conversation with David Linden about his new book on human individuality.

Posted Sep 05, 2020 | Reviewed by Abigail Fagan

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Almost everyone loves to reflect on what makes them special. Johns Hopkins neuroscientist David Linden indulges that impulse in his new book, Unique: The New Science of Human Individuality. He swiftly rejects the “nature vs. nurture” paradigm, replacing it with an exploration of the countless twists and turns that emerge through the relationship between our genes and environment. Linden examines what makes you “you”—and why that matters to the world.

Why did you choose to write about individuality?

About four or five years ago, I found myself single and looking to meet someone on OkCupid. As a scientist, you can’t help but step back and think about the scientific issues at the same time as you’re looking through profiles or trying to make yourself look good. People talked about traits from “I have red hair” to “I’m a typical middle child” to “I despise white chocolate.” I’d think, “How do these traits come about?”

I was there to meet someone and I did—I found my wife there so it was a happy ending—but it also really got me thinking about human individuality and what we know and don’t know about it.

In the book you write, “A good phrase to replace the tired and inaccurate ‘nature versus nurture’ is the more complicated ‘heredity interacting with experience, filtered through the inherent randomness of development.’” What does that mean?

The phrase that everyone knows, “nature vs. nurture,” is so awful and so wrong. The “vs.” implies that what you inherit from your parents must be opposed to your experience in the world, which isn’t true. A lot of times, these things interact. Take the genetic disease called phenylketonuria or PKU. You have to inherit broken copies of a gene that metabolizes the amino acid phenylalanine from both your parents. But it only matters if you eat foods rich in phenylalanine. If you never do, then who cares?

The word “nurture” makes people think too much about family, how your parents took care of you or failed to take care of you. The experience that informs you as an individual is so much broader than that. It’s things like the food your mother ate when she was carrying you in utero. Or the time of year you were born. Or the ambient temperature in the first year of your life.

The last problem is it leaves out one of the most crucial things: pure, random luck. Even at birth, identical twins who have the same DNA don’t come out identical. They don’t look identical, they don’t have identical temperaments, and if you put them in medical scanners their organs aren’t identical. Why is that? It’s because the DNA doesn’t specify the way we develop in excruciating detail with every connection between every cell specified. It gives a vague set of instructions that randomness then acts upon. Randomness is the cherry on top of the hereditary-interacts-with-experience equation.

What’s a surprising source of individual differences?

When you are being carried in utero, and your mom fights off an infection, particularly a viral infection, her body produces molecules called cytokines that are involved in the immune response. Those molecules pass through the umbilical cord, into the fetus, and into the developing nervous system. There are receptors there for those cytokines in the brains of the developing fetus that affect development.

We know for moms fighting off infections in pregnancy, their children have a higher risk of schizophrenia and autism. From animal studies, we have candidate molecules and molecular pathways that may be involved. The evidence from humans is mainly epidemiological evidence, but that’s going to change in the years to come.

What’s a popular misconception about individuality?

Most people think that your birth order is an extremely important determinant of how you behave in the world as an adult. It turns out that when psychologists look at this carefully, it’s just not true. Your birth order is an important determinant of how you behave and interact within your own family, both as a child and an adult. But it’s not as if first children who tend to be the leaders in their family are the leaders on the playground at school or the leaders of corporations. It doesn’t transfer over that way.

How should the science of individuality inform public policy?

There are a lot of people who’ll say, “There’s no point in spending money on public health or education for poor people or people who have a different ethnicity because the traits we care about are entirely heritable, so they’ll never improve.” That’s just not true.

All the cognitive and behavioral traits we know about are not produced by either one gene or a small number of genes. They’re produced by many subtle variations adding up and interacting. A recent study tried to identify the genes that contribute to IQ test scores in Western Europeans. It was a list of over 1,000 genes.

We know from adoption studies that if you adopt poor kids into wealthy homes, they do 15 points better on their IQ test, on average, while their sibling who was adopted into a poor home does not. We know that chronic stress damages the brain, particularly early in life. If you live in a neighborhood with lots of crime and people are always anxious, that impairs your cognitive and emotional development.

Has your research changed your own behavior?

I try to be more aware of when I’m acting on autopilot. I don’t know if I succeed, but I try to do that. The place where it’s most manifest is in my moral and political calculations. If you understand that we’re subject to strong internal motivations and manipulations by external forces, such as our peer group or advertising, you become more compassionate.

For example, a lot of people consider addiction a disease. The heritability of addiction varies across different populations, and it’s about 40 percent among middle-class people. However, BMI is more heritable than addiction. Yet if most people look at someone who is overweight, they don’t say, “You got dealt a rough genetic hand.” They say, “You eat too much and you don’t exercise enough. It’s your own damn fault.” Understanding the statistics of heritability can alter your compassion for people.

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