judith_rich_harris's picture
Independent Investigator and Theoretician; Author, The Nurture Assumption; No Two Alike: Human Nature and Human Individuality
Do genes influence human behavior?

This question bit the dust after a brief but busy life; it is entirely a second-half-of the-20th-century question. Had it been asked before the 20th century, it would have been phrased differently: "heredity" instead of "genes." But it wasn't asked back then, because the answer was obvious to everyone. Unfortunately, the answer everyone gave — yes! — was based on erroneous reasoning about ambiguous evidence: the difference in behavior between the pauper and the prince was attributed entirely to heredity. The fact that the two had been reared in very different circumstances, and hence had had very different experiences, was overlooked. 

Around the middle of the 20th century, it became politically incorrect and academically unpopular to use the word "heredity"; if the topic came up at all, a euphemism, "nature," was used in its place. The fact that the pauper and the prince had been reared in very different circumstances now came to the fore, and the behavioral differences between them was now attributed entirely to the differences in their experiences. The observation that the prince had many of the same quirks as the king was now blamed entirely on his upbringing. Unfortunately, this answer, too, was based on erroneous reasoning about ambiguous evidence. 

That children tend to resemble their biological parents is ambiguous evidence; the fact that such evidence is plentiful — agreeable parents tend to have agreeable kids, aggressive parents tend to have aggressive kids, and so on — does not make it any less ambiguous. The problem is that most kids are reared by their biological parents. The parents have provided both the genes and the home environment, so the kids' heredity and environment are correlated. The prince has inherited not only his father's genes but also his father's palace, his father's footmen, and his father's Lord High Executioner (no reference to living political figures is intended). 

To disambiguate the evidence, special techniques are required — ways of teasing apart heredity and environment by controlling the one and varying the other. Such techniques didn't begin to be widely used until the 1970s; their results didn't become widely known and widely accepted until the 1990s. By then so much evidence had piled up that the conclusion (which should have been obvious all along) was incontrovertible: yes, genes do influence human behavior, and so do the experiences children have while growing up. 

(I should point out, in response to David Deutsch's contribution to the World Question Center, that no one study, and no one method, can provide an answer to a question of this sort. In the case of genetic influences on behavior, we have converging evidence — studies using a variety of methods all led to the same conclusion and even agreed pretty well on the quantitative details.)

Though the question has been answered, it has left behind a cloud of confusion that might not disappear for some time. The biases of the second half of the 20th century persist: when "dysfunctional" parents are found to have dysfunctional kids, the tendency is still to blame the environment provided by the parents and to overlook the fact that the parents also provided the genes. 

Some would argue that this bias makes sense. After all, they say, we knowhow the environment influences behavior. How the genes influence behavior is still a mystery — a question for the 21st century to solve. But they are wrong. They know much less than they think they know about how the environment influences behavior. 

The 21st century has two important questions to answer. How do genes influence human behavior? How is human behavior influenced by the experiences a child has while growing up?

JUDITH RICH HARRIS is a writer and developmental psychologist; co-author ofThe Child: A Contemporary View Of Development; winner of the 1997 George A. Miller Award for an outstanding article in general psychology, and author ofThe Nurture Assumption: Why Children Turn Out The Way They Do.