In the May 20th, 2015 edition of the Huffington Post, Macrina Cooper-White reported on a recently published meta-analysis (analysis of combined studies) of 2,748 twin studies performed in 39 different countries published between 1958 and 2012, which looked at more than 17,000 physical, medical, and psychological characteristics (traits). Cooper-White concluded that the results of this study indicate that the nature-nurture debate “may now be over,” as the study found that nature (genetic) and nurture (environmental) influences “are virtually tied.”
In the original study published online in the prestigious scientific journal Nature Genetics, Tinca Polderman and a group of prominent colleagues concluded that their “results provide compelling evidence that all human traits are heritable.” This conclusion is not new, however, as behavioral geneticists have been making this claim for decades. Polderman and colleagues believed that their meta-analysis of twin method comparisons of reared-together MZ and DZ twin pairs “provides the most comprehensive analysis of the causes of individual differences in human traits,” which “will guide future gene-mapping efforts.”1 (The researchers excluded so-called reared-apart twin pairs from their study.)
Amazingly, the researchers failed to mention that genetic interpretations of MZ-DZ correlational differences are based on the validity of the twin method’s controversial “equal environment assumption” (EEA). The EEA requires twin researchers to assume that reared-together MZ and DZ pairs grow up experiencing roughly equal environments, and that the only factor distinguishing these two types of twin pairs is their differing degree of genetic relationship to each other (100% MZ versus an average 50% DZ). This assumption is obviously false, as the evidence clearly shows that MZ pairs grow up experiencing much more similar environments and treatment than do DZ pairs, and that MZs experience much greater levels of identity confusion and psychological closeness as well.2
Because twin method MZ-DZ comparisons are based on the false assumption that MZ and DZ pairs experience equal environments, it doesn’t matter whether researchers pool together the results of 5 twin studies, 500 twin studies, 2,748 twin studies, or a million twin studies. Like the individual studies, the pooled results for behavioral characteristics can be completely explained by the non-genetic (environmental) influences experienced to a much greater degree by MZ versus DZ twin pairs. Two wrongs don’t make a right, and 2,748 environmentally confounded twin studies pooled together don’t make a genetic finding, at least as it relates to human behavioral differences.3
The classic textbook example of the folly of concluding that correlation equals cause is the finding that ice cream sales correlate with the violent crime rate. Both increase as the weather gets hotter in the summer months, and it obviously would be wrong for an investigator to conclude that eating ice cream causes people to commit violent crime—even if the same results, and accompanying conclusions, are found in 2,700+ pooled studies. The authors of the Nature Genetics study have committed a similar correlational folly by concluding that the greater behavioral resemblance of MZ versus DZ twin pairs is caused by genetic factors, and as usual the popular press follows right along without mentioning that critics have thoroughly exposed this folly in numerous publications going back to the 1930s.4
Regardless of the (predictable) pooled results and conclusions found in the Polderman et al. Nature Genetics study, because the EEA is false, genetic interpretations of all past, present, and future MZ–DZ twin method comparisons in psychiatry and in the social and behavioral sciences in general must be rejected outright. The classical twin method as a measure of “heritability” and of genetic influences on psychiatric disorders and other behavioral characteristics remains one of the great pseudoscientific methods of our time, and will eventually be added to the list of discarded pseudosciences where we now find alchemy, craniometry, and mesmerism.
However, because false genetic theories and techniques are needed by powerful economic and political interests, it is unlikely that the twin method will be abandoned in the absence of significant social and political change.
* * * * *
1. Polderman, T. J. C., 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, published online 5/18/2015. doi:10.1038/ng.3285
2. Joseph, J., (2004), The Gene Illusion: Genetic Research in Psychiatry and Psychology under the Microscope, New York: Algora; Joseph, J., (2015), The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences, New York: Routledge.
3. A “confound” is an unforeseen or uncontrolled-for factor that threatens the validity of conclusions researchers draw from their studies. Confounding occurs when the association between two variables is caused by a third variable that influences both. In the twin method, the more similar environments experienced by MZ twin pairs confound genetic interpretations of MZ-DZ correlational differences.
4. See Joseph, 2004, Chapter 2.