Being Raised Together Doesn't Make Us The Same
What sibling differences tell us about family influences
Posted Jun 24, 2019
Recall from the first post in this series that behavior genetics broadly sorts the potential causes of differences between people into two bins: genes (your DNA) and the environment (everything else). The environment is often split into two categories: the shared environment and the unique environment.
The second law of behavior genetics is that the influence of the shared environment is smaller than the influence of either genes or unique environments. This is often misinterpreted to mean that parents don't influence their children. But it actually tells us nothing about the influence of objectively shared experiences, like parents, schools, or cultures. To understand why, let's break down what behavior geneticists mean when they talk about "the shared environment."
Estimating shared environmental influences
When behavior geneticists separate causes of individual differences into genetic, shared environment, and unique environmental categories, we don't necessarily directly measure any of those influences. Rather, these constructs are estimated from comparisons of people who share different amounts of genes and environments. By definition, shared environmental influences appear when people who were raised together are similar to one another, regardless of how genetically similar they are. If shared environmental influences are present, and there is no influence of genes, we would expect any children who are raised together to be equally similar to one another, regardless of whether they are twins (identical or fraternal), non-twin siblings, half-siblings, cousins, or even adopted siblings. The construct that we label "shared environment" is literally defined in our statistical models as: stuff that is the same - correlated at 1.0 - among all members of a family.
On average, identical twins (correlated at 0.6) are more similar than fraternal twins (correlated at 0.3). If the shared environment played a substantial role in adult outcomes, then identical twins would be less than twice as similar as fraternal twins. Or, rephrased using the neat trick of Falconer's formula with twin correlations for estimating the shared environment:
- shared environment = (2 x fraternal twin correlation) - identical twin correlation
- shared environment = (2 x 0.3) - 0.6 = 0
Even when using more advanced statistical models, the average estimate for the influence of the shared environment across all human individual differences is relatively low, at 17%.
Shared environmental influence is higher in childhood
In adulthood, most traits show little influence of shared environmental factors, but the same is not true in childhood. Shared environmental influences tend to be higher in childhood, and decrease over time, as children gain more independence and opportunity to experience environments (food, friends, activities) that they choose for themselves, rather than those that are chosen for them by their parents. One large twin study to demonstrate this effect was a meta-analysis of the heritability of and environmental influences on cognitive ability test scores. Around 9-years-old, the influence of the shared environment was 33%. That is, experiences that are the same among children who are raised together accounted for one-third of the differences among all children in general cognitive ability. However, the influence of the shared environment decreases to 16% by 17-years-old. The average 17-year-old has much more say in how they spend their time, including effort toward, say, studying for school, than the average 9-year-old. So as independence increases, there is understandably less of a role for any shared environmental influences, as they give way to the influence of unique experiences.
Shared environmental influences aren't the same as family influences
Shared environmental influence in behavior genetic research does not come from testing whether actual measures of presumably shared environments are correlated with outcomes. Rather, shared environmental influence is estimated by examining whether being raised together makes outcomes more similar, regardless of genetic similarity. Anyone with siblings or multiple children can attest that children raised together experience the same parents, school, or culture in unique ways. Every parent has a different relationship with each of their children. An objectively shared experience (such as parents divorcing) may be reacted to quite differently by each child (even if they are identical twins). In that case, the impact of the objectively shared experience is operating as a part of the unique environment, rather than as a shared environment, because its impact on the individual is unique. It's not that family doesn't matter; rather, we all perceive and react to "shared" experiences in different ways.
Previously: Everything’s a little bit heritable.
Coming up: The third and fourth laws of behavior genetics.
Polderman, T. J., Benyamin, B., De Leeuw, C. A., Sullivan, P. F., Van Bochoven, A., Visscher, P. M., & Posthuma, D. (2015). Meta-analysis of the heritability of human traits based on fifty years of twin studies. Nature Genetics, 47(7), 702-709. https://www.nature.com/articles/ng.3285
Haworth, C. M., Wright, M. J., Luciano, M., Martin, N. G., de Geus, E. J., van Beijsterveldt, C. E., ... & Kovas, Y. (2010). The heritability of general cognitive ability increases linearly from childhood to young adulthood. Molecular Psychiatry, 15(11), 1112-1120. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2889158/