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In the early 1990s, a psychologist named Robert Plomin began a wide-ranging study whose results are upending everything we thought we knew about the roots of human behaviour. He began to study twins. He wasn't the first in the field, but today he is the most influential.

Twins come in two types: identical, which means they share all their genes; and fraternal, which means they share no more genes than any other siblings. They present a natural experiment in why people differ, and the influences of nurture versus nature. Bottom line: Genetics is important everywhere we look – not just in height and hair colour but also in behaviour. "A good general guess is that genetics explains 50 per cent of the differences in people in terms of personality, vocational interest, depression," Dr. Plomin says. "That's a huge amount of explanatory power."

Today, spectacular advances in molecular genetics are producing an explosion of new information about how our genome shapes us. Harvard psychologist Steven Pinker predicts that these findings will fundamentally reshape our understanding of human nature. They may also reshape our thinking on everything from parenting and education to broader social policy.

No doubt you've read those stories about identical twins who were separated at birth but grew up to be uncannily alike. My favourite is the story of Jack Yufe and Oskar Stohr, separated when they were six months old. Jack, raised in Trinidad as a Jew, later joined Israel's navy as an officer. Oscar, who grew up in Nazi Germany, was Catholic and a member of the Hitler Youth.

But the similarities were even more striking than their differences. Both were hot-tempered, impatient and demanding. They preferred to read books back to front. They flushed toilets before and after using them. Both had dreams about killing the other in war. When they met in later life, both showed up wearing white sports jackets and wire-rimmed glasses.

Here's just some of what the new genetic science has shown:

* Serious disorders once thought to be wholly psychological in origin are now known to be organic. Autism was thought to be a developmental disorder caused by bad parenting. Schizophrenia was blamed on "refrigerator mothers." Now we know from twin studies that both autism and schizophrenia have a strong genetic component, as do learning disabilities.

* The effects of family socialization on your children's personalities, as Brian Boutwell and Razib Khan write in Quillette, "are not large, not prominent and not pronounced." What you do (or don't do) to your kids has no influence on their innate intelligence, their temperament and personality traits, or their interests. Spank or don't spank, read to them or not – it really doesn't matter. You can, of course, make a difference if you save up all your money to send your kid to Harvard, or try to teach your antisocial miscreant anger-management techniques. But you can't change who they are. Your greatest influence on their lives was the moment you rolled the dice and bequeathed your and your partner's genes to them.

*Intelligence (as Dr. Plomin and others wrote in an influential piece in Nature) is "one of the best predictors of important life outcomes such as education, occupation, mental and physical health and illness, and mortality." Intelligence, one of the most heritable behavioural traits, is also an important factor in class differences. Intelligent people are healthier, happier and stay married longer. They are also likely to marry each other and produce intelligent children. The implications for inequality and social mobility are significant.

It's easy to see why this stuff is so explosive. It sounds so fatalistic! The idea of innate differences is anathema to many people who value equality. If genes are so important, what happens to our efforts to create social justice? What happens to our notions of social reform progress? There's also the grim spectre of the past. Genetic determinism has been linked with some of the vilest chapters in recent history.

No wonder many people – especially social scientists, feminists, and progressive politicians – think behavioural genetics is a Pandora's box that should be slammed shut as soon as possible. They remain heavily invested in cultural determinism – the idea that your environment, not your origins, makes you who you are, and also that the right social policies can significantly change the outcomes.

But genetic denialism has its own risks. One risk is that by not acknowledging the importance of heritability, a lot of social science research is misleading and useless. And many of the policies it has inspired won't work.

Today the social sciences face a deepening crisis of legitimacy – largely because social scientists, who are overwhelmingly liberal, can't bring themselves to acknowledge what's staring them in the face. Yet good social policy depends on it. Dr. Plomin believes this is especially true in education, which should be both more effective and more humane. "It does poor service to social change to subordinate truth to politics," he says.

With the revolutionary new techniques that have been developed for exploring the human genome, the cascade of findings in behavioural genetics will be increasingly difficult to ignore. They'll be controversial, but also liberating. Meanwhile, you can focus on the good news: Whatever's wrong with your kids is probably not your fault.

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