Twin Studies If a genetic theory of behavior depends on twin study data, the theory is in serious trouble. Based on twin study results, biopsychiatry estimates MD heritability in the 30%-40% range. (I make a distinction between psychiatry and biopsychiatry, while being aware that biological and genetic approaches currently dominate psychiatry. The psychiatric genetics field is a major component of biopsychiatry.) Genetic theories in psychiatry are based on studies using reared-together twin pairs. Other than anecdotal reports on individual pairs, there are no reared-apart twin studies in psychiatry, even though psychiatric texts at times say that there are.
Psychiatric twin studies use the classical twin method, which compares the concordance rates or behavioral correlations of reared-together MZ (monozygotic, identical) versus reared-together same-sex DZ pairs (dizygotic, fraternal). MZ pairs are assumed to share 100% of their segregating genes, whereas DZ pairs are assumed to share on average 50%. The results of MD twin studies show that MZ pairs resemble each other more for MD compared with same-sex DZ pairs at a statistically significant level. I will designate this finding “rMZ > rDZ” (with r representing the behavioral correlation).
All sides of the genetics of depression debate expect a twin study finding of rMZ > rDZ. The main disagreement centers on how this expected-by-all finding should be interpreted.
Genetic interpretations of rMZ > rDZ require acceptance of the long-controversial MZ-DZ “equal environment assumption,” also known as the “EEA.” According to the EEA, MZ and same-sex DZ pairs grow up experiencing roughly equal environments, and the only behaviorally relevant factor distinguishing these pairs is their differing degree of genetic relationship to each other (100% vs. an average 50%). This key assumption is obviously false, however, since when compared with same-sex DZ pairs, MZ pairs grow up experiencing
Much more similar treatment by parents and others, including being dressed alike More similar physical and social environments, including spending more time together, attending classes together, and having common friends and peer groups More similar treatment by society due to their sharing a very similar physical appearance A greater tendency to model their behavior on each other Identity confusion and a much stronger level of emotional attachment to each other Most modern twin researchers concede the point that MZ environments are more similar. For example, in a 2014 article by criminology twin researcher J. C. Barnes and colleagues, ironically written in defense of twin research, the authors properly recognized, “Critics of twin research have correctly pointed out that MZ twins tend to have more environments in common relative to DZ twins, including parental treatment…closeness with one another…belonging to the same peer networks…being enrolled in the same classes…and being dressed similarly.”
Despite recognizing that MZ and DZ twin pairs grow up experiencing very different environments, twin researchers have used eight different arguments in support of the EEA. In my forthcoming book Schizophrenia and Genetics: The End of an Illusion (Routledge, 2023), I examine each of these eight arguments and show that none holds up (a partial examination of these arguments can be found here). Because the EEA is false, the results of a psychiatric twin study finding rMZ > rDZ can be explained by non-genetic factors. Decades of studies designed to “test” the EEA have failed to alter this basic conclusion.
In a 2000 MD “review and meta-analysis” based on twin study data, leading genetic researchers Patrick Sullivan, Michael C. Neale, and Kendler calculated a 37% MD heritability estimate based on the greater MZ versus DZ resemblance for depression. Sullivan and colleagues sensibly did not claim that MZ and DZ environments are equal, and like most authors of the six depression twin studies they analyzed, they sidestepped the twin method’s unequal environments problem by defining the EEA in its “trait-relevant” form: “The critical equal environment assumption,” they wrote, “posits that monozygotic and dizygotic twins are equally correlated in their exposure to environmental events of etiologic relevance to major depression” (emphasis added).
A principle of science, however, is that the burden of proof falls on people making a claim, not on their critics. Therefore, MD twin researchers using this trait-relevant definition of the EEA—and not their critics—are required to identify the specific and exclusive trait-relevant environmental factors involved in a diagnosis of major depression. Until this happens, and until they then determine that MZ and DZ pairs were similarly exposed (or not exposed) to these factors, the EEA as conceptualized by Sullivan and colleagues fails completely. Because the EEA is false, MD twin study and twin-study-based meta-analysis results cannot be interpreted genetically.