Comments about Frank Miele's Article "The Revival of Human Nature [Not Equal to] the Denial of Human Nurture."

Article excerpt

The Gene Illusion: A Critique of Frank Miele

Of the many problems with Miele's article, I would like to focus my comments on his reliance on behavior genetic research, since a major portion of my recent book, The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope, is dedicated to a critical analysis of this body of research. (1)

Like Steven Pinker in The Blank Slate and Matt Ridley in The Agile Gene, Miele's argument rests mainly on the results of reared-together twin studies, and studies of reared-apart twins. (2) However, twin research is plagued by bias, methodological problems, and a reliance on untenable theoretical assumptions. The main tool of behavior genetics and psychiatric genetics is the "classical twin method," more commonly known as "the twin method." The twin method compares concordance rates or correlations of reared-together identical (also known as monozygotic or MZ) twins, who share 100% genetic similarity, versus the same measures of reared-together same-sex fraternal (also known as dizygotic or DZ) twins, who average a 50% genetic similarity. Based on the assumption that both types of twins experience equal childhood and adult environments, known as the "equal environment assumption" or "EEA," twin researchers attribute a significantly higher resemblance among identical versus same-sex fraternals to genetic factors.

However, as I documented in The Gene Illusion, there is overwhelming evidence that identical twins experience much more similar environments than fraternals, and, perhaps more important, identicals experience a stronger psychological bond and more often experience identity confusion. (3) Twin researchers often concede these points, yet continue to uphold the validity of the twin method on the basis of two claims. The first is that, although identical and fraternal twin environments are different, critics must identify the "trait-relevant" environmental factors for which identical and fraternal twins experience dissimilar environments. However, because a basic tenet of science holds that the burden of proof falls on the claimant, not on critics, (4) twin researchers themselves bear the burden of proof for showing that identical and fraternal twins are not differentially exposed to potentially relevant environmental factors. Moreover, although faced with a similar problem, twin researchers and behavior geneticists do not make the "trait relevant" argument when discussing potential environmental confounds in family studies. In this case they are willing to concede that, because family members share a common environment ("trait-relevant" or not), one cannot draw valid conclusions in favor of genetics on the basis of the family resemblance of a trait.

The second claim in defense of the twin method is that identical twins "create" more similar environments for themselves by virtue of their greater genetically-caused similarity of behavior. (5) Therefore the twin method's validity, according to twin researchers, is based on determining why--not whether--identical twins experience more similar environments than fraternals. Twin researchers and popularizers of their work, however, fail to understand that the reason identical twins experience more similar environments than fraternal twins, be it environmental or genetic, is completely irrelevant in assessing the validity of the EEA. The only relevant question is whether--not why--identical twins experience more similar environments. Moreover, the "twins create their environment" argument is circular because the evidence that twin behavioral similarity is caused by genetics is implicitly derived from previous twin studies.

Therefore, like family studies, the twin method is unable to disentangle the possible role of genes and environment on psychological trait variation or psychiatric disorders. There is, in fact, little reason to accept that the twin method measures anything other than the more similar treatment, greater environmental similarity, and closer psychological association experienced by identical versus fraternal twins. …