The 50-0-50 Rule: Why Parenting Has Virtually No Effect on Children

“Do parents have any important long-term effects on the development of their child’s personality? This article examines the evidence and concludes that the answer is no.” Thus begins Judith Rich Harris’s groundbreaking 1995 Psychological Review article, “Where is the child’s environment? A group socialization theory of development.”

Judith Rich Harris is one of the most unconventional heroes of behavior genetics. In 1960, she was a graduate student in psychology at Harvard University. After receiving her Master’s degree, she was dismissed from the program by the then acting department chair, George A. Miller, who thought Harris was not smart enough to earn a Ph.D. While supporting herself by writing psychology textbooks 35 years later, Harris worked on her group socialization theory of development and published it in the prestigious academic journal Psychological Review. In 1997, her article won an award from the American Psychological Association, the George A. Miller Award for an Outstanding Recent Article in General Psychology. Yes, as Harris herself puts it, God has a sense of humor.

In her 1995 article, and then in her 1998 book The Nurture Assumption: Why Children Turn Out the Way They Do, Harris methodically demolishes the universally-held assumption that how parents raise their children is a major determining factor in how they turn out. Harris instead argues that parental socialization has very little effect on children because they are mostly socialized and influenced by their peers. While Harris’s conclusion was enormously controversial and widely condemned by politicians and the media alike, it is in fact corroborated by behavior genetic research.

Behavior geneticists decompose total variance in personality and behavior into three components: heritability (genes), shared environment (everything that happens within the family that makes siblings from one family similar to each other but different from those from another family), and unshared environment (everything that happens within and outside the family that makes siblings from one family different from each other). Behavior geneticists contend that the rough rule of thumb when it comes to the determinants of child development is 50-0-50, that is, roughly 50% of the variance in personality, behavior, and other traits is heritable (influenced by genes), roughly 0% by the shared environment (what happens within the family and is experienced by all siblings), and roughly 50% by the nonshared environment (what happens inside and outside of the family, not shared by siblings).

Of course, the precise breakdown among the three components varies by the trait in question, and also by the population used to derive the estimates. For example, intelligence has greater heritability than most personality traits, and is roughly 80% heritable (determined by genes). Another problem with the behavior genetic method outlined above is that the category of “unshared environment” is a residual category, which includes not only all the genuine effects of unshared environment but all the errors and unmeasured effects which do not fall into the first two categories. It also captures how individuals with different genetic predispositions react to the same environment differently (gene x environment interaction). However, for most personality and behavioral traits, the 50-0-50 rule roughly holds.

Harris’s work highlights the importance of the nonshared environment (in particular, peer socialization) on child development, and partly explains why siblings who share half their genes and raised by the same set of parents within the same family can often be very different, as different oftentimes as children from different families. Of course, contrary to how the media portrayed (and viciously attacked) Harris’s work, it decidedly does not mean that parents are not important for children’s development. On the contrary, it means that parents are enormously important because children receive 100% of their genes from their biological parents and some of the unshared environment is provided by the parents. It simply means that, within broad limits, how parents raise and socialize their children may not be very important to adult personality. It also explains why adopted children often turn out to be very similar to their biological parents and not at all like their adoptive parents.

Children do greatly resemble their parents in their personality, values, and behavior. But it is mostly because they share common genes, not because the parents raised the children in certain ways. As the late great behavior geneticist (and an old friend of mine) David C. Rowe puts it: “Parents are often given too much credit for children who turn out well, and too much blame for children who turn out poorly. The source of causal influence is not in rearing variation, but in the genes and in unshared environmental variation.”

P.S. Here is a recent New York Times article, which makes the same point about the relative importance of genes, and the relative unimportance of parenting, for how children turn out, featuring the research of PT’s own Nancy L. Segal. Thanks once again to Jay Belsky for alerting me to this article and for very useful discussions.