In an earlier blog post I mentioned that until very recently there were essentially three types of studies researchers use to differentiate between the effects of genes and the effects of parental socialization. I have mentioned two of those study designs – adoption studies and twin studies. There are various methodological variants on the twin study which I will not get into.
However I have not mentioned the third study design used to estimate the effects of genes. This is the so-called combination adoption twins study. This sort of study design requires that the researcher finds identical (MZ) and/or fraternal (DZ) twins who have been raised in different families since birth for at least since very early in their life. Obviously, it’s very difficult to find these kinds of people and so there are very few studies of twins raised apart.
Perhaps the most famous study of this kind is the Minnesota study of twins reared apart (aka MISTRA). Over perhaps a decade or more researchers at the University of Minnesota recruited MZ and DZ twins who reported that they had been separated at birth or a bit later. The Minnesota group recruited twins from all over the country and in some cases from outside of the United States. Twin pairs were brought to Minnesota to answer dozens of questions gauging their beliefs, skills and personality skills. The results were stunning.
The Minnesota team reported many of their most important findings in a classic 1990 article in Science magazine. MZ twins who had been raised in separate families nonetheless were quite similar to each other in a variety of ways. The Minnesota group also compared monozygotic twins reared apart with monozygotic twins reared together. For example consider IQ. Correlations between the IQs of MZ twins reared apart were quite high, albeit smaller than the IQs of MZ twins reared together. Among the 48 pairs of MZ twins reared apart, WAIS full test scores correlated at 0.69. Among the 40 pairs of MZ twins reared together, WAIS full test scores correlated at 0.88. This result provides persuasive evidence that IQ has a very large heritable component.
Limitations with studies of twins reared apart?
Theoretically, the adopted twins studies provide the most robust test one can imagine. Think about what these studies have potentially found. The idea is that you take two genetic clones, put them in different families and then 40 years later see how similar they are. If the twins are very similar then you can say with a very level of confidence that genes are making a big difference. On the other hand if the twins aren’t that similar you can say that genetic influence is weak. Of course the reality is somewhere in between but the upshot of the Minnesota study of twins reared apart as well as other similar studies such as those in Sweden is that genetic influence is substantial.
This is not to say that there are not limitations or at least questions about these sorts of studies. For example, some researchers questioned whether the twins in the Minnesota study had been raised in households that were entirely distinct from one another. You could have for example two twins whose parents had died adopted by different members of the same extended family. Such twins adopted in different nuclear families but the same extended family may have remained in close contact for much of their lives. In defense of the Minnesota study, researchers did collect data on the amount of time that the twins spent together and the age at which they had been separated. Generally the researchers found that age of separation and the amount of time that the twins had spent together made little or no difference on similarity in how they turned out.
The Minnesota study of twins reared apart may also suffer from selection bias. When doing a study of twins reared apart, you don’t have the luxury of randomly choosing your sample from your population of interest. Instead when you’re looking for such a rare population as identical twins separated at birth or in childhood, you’re going to take anybody you can get. The problem with this is that the twins who end up volunteering for your study may in fact be twins who tend to be more similar to each other. Twins who are distant or dislike each other may be less likely to volunteer for such a study. For this reason, correlations between the twins in the study on any given trait may be artificially inflated relative to what you would find if you had a truly representative sample. Nevertheless the findings of Minnesota study cannot be easily dismissed. Even if we allow for the fact that some of the twins in the Minnesota sample did in fact spend a good deal of time together prior to the study, the conclusion and the results are still striking.