It’s Time to Abandon the “Classical Twin Method” in Behavioral Research
(First revision 8/17/2020. Second revision 12/23/2020, based on small stylistic changes, and adding a new introductory paragraph and a paragraph about a 2018 study by Segal and colleagues)
In this article I will argue that studies using the "classical twin method" should be abandoned in the social and behavioral sciences. I will attempt to show that the main assumption these studies are based on is false—an argument I have developed in books, peer-reviewed articles, and online articles since 1998.
Twin studies supply the "scientific evidence" most often cited in support of the claim that human behavioral differences are strongly influenced by heredity. Yet genetic interpretations of twin studies of behavior, including areas such as IQ, personality, criminality (antisocial behavioral), schizophrenia, and depression are based on the acceptance of highly questionable or even false assumptions. I am compelled to keep writing about this because these studies have not gone away, despite the critics’ airtight arguments that they should have gone away a long time ago.
similar physical appearance
This means that twin researchers and their critics don't have to argue anymore about whether MZ and DZ environments are different, since almost everyone now agrees that they are different. Twin researchers nevertheless continue to maintain that the EEA is valid based on four arguments, which I describe and deconstruct below.
Argument A fails because it is a sleight-of-hand "heads I win, tails you lose" circular argument that twin researchers cannot lose, because they count both the "nature" and "nurture" sides of the behavioral coin as nature (genetic).
To identify only one of several problems with this argument, let's suppose that the environmental null hypothesis—which states that there are no genes for behavior—is true. In this case mating patterns would have no direct genetic influence on human behavior, and MZ > DZ would be completely caused by non-genetic factors. The claim that non-random mating patterns lead to a "modest understatement of the role of genes" circularly assumes in advance that the environmental null hypothesis is false. A twin study, however, is an experiment designed to test whether the environmental null hypothesis is false. The findings of this experiment cannot be based on a built-in assumption that it is false, especially since this assumption, once again, is based largely on genetic interpretations of previous twin studies. Theoretical sleight of hand scores a hat trick.
Argument D: "MZ Twins Correlate Similarly on Psychological Tests Regardless of Whether They Were Reared Together or Reared Apart"
The final argument is based on the results of the tiny handful of TRA (twins reared apart) studies, and the claim that MZ pairs behave similarly regardless of whether they were reared together or reared apart. People making this claim argue that this supports the EEA, because it shows that growing up in the same family does not lead twins to behave more alike. However, the argument does not take into account the numerous non-familial environmental factors that reared-together and reared-apart twins both experience. More importantly, TRA studies are greatly flawed. The massive flaws and major biases found in these studies, including the famous Minnesota study, are described in detail HERE and HERE. As seen HERE, most MZ pairs found in TRA studies were only partially reared apart. For these reasons, TRA study results cannot be used to validate genetic interpretations of MZ > DZ.
Like the first three arguments, Argument D fails to support the EEA, and the only remaining relevant question in assessing the EEA's validity is whether—not why—MZ and DZ environments are different.
Problem #3. Although twin researchers do not recognize it as such, nine decades of twin studies have produced the greatest combined "test" of the EEA ever seen. These studies have consistently shown that twin pairs experiencing similar environments and high levels of identity confusion and attachment—MZs—behave much more alike than do pairs experiencing less similar environments and lower levels of identity confusion and attachment—DZs. The obvious conclusion in this EEA-test study is that MZ > DZ can be explained on environmental (non-genetic) grounds.
Problem #4. Cognitive neuroscientist Chris Chambers has described several major problem areas in the research/publication process in psychology. One of these is "hidden flexibility," which refers to researchers' ability to change various aspects of their study after reviewing their data, but before submitting their paper for publication. Under the current system in psychology, and presumably in other behavioral science areas as well, undetected "questionable research practices" may be common.