During the past two decades, psychologists proved two astonishing things:

A large part of our personality—our happiness, intelligence, religiosity, etc.—is determined by our genes,
How your parents raised you plays no role in your personality.

I learned that in Pr Paul Bloom’s Introduction to Psychology. I was shocked, so I did some research. It’s apparently true. Here’s what I found.

Man in Gray Shirt Holding Baby in White Onesie

Many scientists have tried to understand whether our traits were more influenced by our genes (or “nature,” in this case, they’re called “innate” or “hereditary traits”) or our environment (or “nurture” because they’re “acquired”).

The environment can be divided into two parts:

Shared environment: what different family members share. For instance, whether you had family meals with your parents or not.
Non-shared environment: everything else. So if you're different from your brother because you won the lottery one day, that's your non-shared environment.

One of the most studied traits is intelligence, defined and measured by IQ tests. Scientists want to know if people who score high on IQ tests do so because of their genes, shared environment, or non-shared environment.

(Note: IQ tests have many issues, they probably don’t measure “intelligence” as we commonly understand it, but that’s not the point of this article...)

To do so, scientists look at siblings and measure the correlations between their traits and their parents’ or biological parents’. Studies especially focus on twins—even more on separated twins—and adopted children to measure the influence of the genes and the environment.

Their results:

Identical twins raised separately are surprisingly alike,
Identical twins differ too much the same extent whether they are raised together or apart,
Adoptive siblings are as unalike in personality as non-related children,
Adopted children’s personality is more correlated with their natural parents’ (who had no part in their upbringing) than their adoptive parents’.

Judith Harris first explained this in her 1998 book “The Nurture Assumption: Why Children Turn Out the Way They Do; Parents Matter Less than you Think and Peers Matter More” and then confirmed by numerous studies the world.

In the case of IQ, depending on the studies, between 57% and 80% of this trait can be attributed to your genes. And almost 0% to your shared family environment! Your non-shared environment influences the rest. Harris calls this the “50-0-50” rule: roughly 50% of the variance in personality is influenced by genes, 0% by the shared environment, and 50% by the non-shared environment.

So you’re largely shaped by your genes and not by your family. Parents and parenting practices don't matter. But your personal experience outside your family does.

This was a shocking discovery for me. Even more surprising: the heritability of IQ increases with age! I thought that genetic influences on personality would decrease with age as one becomes more independent and develops their own personality. But it’s exactly the opposite. From Wikipedia: “One proposed explanation is that people with different genes tend to seek out different environments that reinforce the effects of those genes.” This is a terrible example of the Matthew effect:

For to every one who has will more be given, and he will have abundance; but from him who has not, even what he has will be taken away.—Matthew 25:29, RSV.

According to Harris, in the non-shared environment, the most important are your friends: “The world that children share with their peers determines the sort of people they will be when they grow up.”

Motivational speakers repeat that “You are the average of the five people you spend the most time with.” They’re (partly) right.

Harris gives the example of children of immigrants who learn the language and the native accent of their home country with their classmates but often can’t speak their mother tongue properly. They identify with their classmates rather than their parents, modify their behavior to fit with their peer group, and this ultimately forms their characters.

So if parenting doesn’t matter, why are parents nice to their children? For the same reason, you treat your partners, friends, and parents nicely: because you love them, not because you want to mold their personalities.

Some personal traits that seem totally acquired and cultural, such as religiosity, are also inherited. Like IQ, genetic factors become more important in determining how religious a person is with age!

So people who carry a certain “religiosity” gene (also called “God gene”) are more likely than average to become or remain religious. Religiosity, like IQ, follows the 50-0-50 rules; your genes influence about half. And because religious people have more children, the religiosity gene will eventually predominate in our societies.

Many other traits have been proved to be influenced by your genes, such as happiness, belief in the supernatural, obedience to authority, conservatism, and susceptibility to ceremony and ritual.

As Paul Bloom concludes in his lesson:

I think there's a lot more research to be done in this area and we might discover that the conclusions that I'm confidently raising right now turn out to be false and that there's more going on. But at this point, we have to live with one of the surprising conclusions that have come from behavioral genetics and from psychology more generally, which is that parenting doesn't matter anywhere near as much as people believe it does.

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