Over the last 30 or 40 years, there has been a considerable amount of research — perhaps two dozen studies — examining whether environmental similarity biases the results of twin studies. The vast majority of this research finds little to no evidence that twin studies are biased in this regard. Most twin studies have cited this research in support of what is known as the equal environment assumption or EEA for short. The equal environment assumption is perhaps a misnomer because it doesn’t mean that we assume that MZ twins share environments that are as similar as DZ twins. Instead, the equal environments assumption simply states that environmental similarity between twins does not have much of an impact on trait similarity.
There are however some gaps in the research investigating the EEA and that’s where my dissertation and forthcoming book came in. In my dissertation, I attempted to test the EEA in a more comprehensive fashion then had been done previously. Using a nationally representative sample of twins, I tested the impact of a wider array of environmental similarity measures. I also looked at the impact of all of these environmental similarity measures on genetic estimates for a wide variety of outcomes that social scientists are interested in. So what did I find? When controlling for a wide variety of measures of environmental similarity I found that the bias affecting twins studies is perhaps larger than many behavior geneticists like to believe but also smaller than many critics of behavior genetics like to believe.
Let me try to explain what I did in my dissertation in layman’s terms. What I essentially did was to find MZ twins and DZ twins whose parents and peers treated them with the same level of similarity at least in so far as could be measured. Consider a concrete example. Most MZ twins are in good contact with each other. Meanwhile DZ twins are less likely to remain in good contact. Part of what I did in my dissertation was to find the DZ twins who do remain in close contact like many of the MZ twins and then to compare that subset of the DZ group with the MZ group. The idea here again is to eliminate any effects of environmental similarity. In some cases when I controlled for environmental similarity, the apparent effects of genes declined dramatically. But in many cases the effects of genes were robust to the inclusion of controls for environmental similarity. This means that when I compared the DZ twins who spend a lot of time together and had very similar experiences with the MZ twins, I still found the same genetic effects as would have been found in the standard twin study that didn’t control for environmental similarity.
One thing that we can conclude is that whatever bias introduced by environmental similarity is not large enough to invalidate the central conclusion of twin studies. The overall conclusion reached by behavior geneticists from twin studies is that nearly every trait you can imagine is heritable. The general idea is that the effects of genes pervade our life and explain a lot of the differences between us. My dissertation added a caveat to that conclusion: perhaps genetic effects are not quite as large as some of the twin studies lead us to believe. Nonetheless, genetic effects are pervasive and often substantial.