Troubling Twin Studies

©: lightwise |

©: lightwise |

In 2004, Paula Bernstein received a call from the agency where she had been adopted. The message was that she had a twin who was looking for her. Elyse Schein—Paula’s twin—had been trying to find out information from the adoption agency about her birth mother when she learned she had a twin sister. They met for the first time in a café in New York City. They talked through lunch and dinner about the 35 years they had been apart. Among the things they discussed was why they were separated at five months old. It seems they were part of a one-of-a-kind study on nature versus nurture of twins separated at birth.

All Things Considered” said the study was headed up by Peter Neubauer, a noted child psychiatrist and Viola Bernard, a child psychologist. The study ended in 1980, a year before the state of New York began requiring that agencies keep siblings together. Neubauer realized that public opinion would be against the study so he didn’t publish it. The data and results have been sealed until 2066 and placed in an archive at Yale University.

The sisters tried to reach Neubauer, but he initially refused to speak with them. Eventually he did agree to meet with them. Paula hoped he would apologize for separating them. Instead he showed no remorse and offered them no apology.

Of the 13 children involved in his study, three sets of twins and one set of triplets have discovered one another. The other four subjects of the study still do not know they have identical twins.

Since the 1920s, researchers in psychology and psychiatry have used twin research (but not the above method) to assess the potential for genetic factors to underlie psychological traits and psychiatric disorders. In the Introduction to his new book, The Trouble with Twin Studies, Jay Joseph noted how this research method is seen as a scientifically valid research method that provides an ideally suited “natural experiment” to assess the relative importance of heredity and environment. According to one estimate, about 800,000 twin pairs had been studied by 2009. “In almost all cases these studies are based on twin pairs reared together in the same family, while an extremely small yet influential handful of studies, twin pairs were said to have been reared apart in different families.”

MZ (monozygotic, identical) twins have 100% of the same genes, where DZ (dizygotic, fraternal) twins share around 50%. Twin researchers have argued that the greater similarity between MZ pairs for behavioral traits and disorders (physical and psychiatric) than same-sex DZ pairs is caused by the greater genetic resemblance of the MZ pairs. Therefore, twin researchers reason, “the trait or the disorder has an important genetic component.” Underlying this is the basic assumption of the twin method, that MZ and DZ twin pairs experience equal environments.

Yet, even twin researchers have concluded that MZ pairs experience more similar environments than DZ pairs. Jay Joseph quoted twin researchers as saying, “the evidence of greater environmental similarity for MZ than DZ twins is overwhelming.” Nevertheless, twin researchers have perpetuated the twin method by using circular reasoning, denying ignoring or downplaying the evidence that MZ and DZ environments are different or changing the definition of what constitutes an equal environment.

In an article that was also titled: “The Trouble with Twin Studies,” Joseph noted how the circular argument in effect says that identical pairs “create” more similar environments because they are more similar genetically. When defending the validity of the twin method, modern twin researchers “refer to the premise in support of the conclusion, and then refer back to the conclusion in support of the premise, in a continuously circular loop of faulty reasoning.”

Another tactic used to support the twin method is to first agree that identical twins grow up in more similar environments than fraternal twins. But then proponents say that it has to be demonstrated that identical and fraternal environments differ in ways that are relevant to the trait in question. If that cannot be done, then they argue the equal environment assumption is valid for that trait.

The bottom line is this: despite being cited in countless textbooks, scholarly journal publications, and popular books and articles, the little-disputed finding that identical pairs experience much more similar environments than fraternal pairs means that non-genetic factors plausibly explain twin method results. The fact that psychiatric twin studies continue to be cited in support of genetics, largely uncritically, speaks volumes about the scientific status of psychiatry in the 21rst century. Psychiatry’s acceptance of twin studies is even more remarkable in the context of the decades-long failure of molecular genetic research to uncover genes that investigators believe cause psychiatric disorders [see Joseph’s article, “Five Decades of Gene Finding Failures in Psychiatry”]—research that is based largely on genetic interpretations of the results of psychiatric twin studies.

There are a handful of studies whose twin pairs were supposedly reared apart from one another in different families. Advocates of twin studies assert that all behavioral similarities between reared-apart MZ twins (known as MZA pairs) must be the result of their 100% genetic similarity, since they have not had any environmental similarities.  Joseph said in “Studies of Reared-Apart (Separated) Twins: “Twin researchers and others view this occurrence as the ultimate test of the relative influences of nature (genes) and nurture (environment).”

Yet there were a variety of problems with these twins reared-apart (TRA) studies. Many twins experienced late separation and many twins were reared together in the same house for several years. Others had regular contact and/or a close emotional bond with each other. In one classic TRA study, twins separated as late as 9-years-old, or for only five years during childhood were counted as “separated twin pairs.” The same study counted a pair living next door to one another, brought up by different aunts, as a separated pair.

Far from being separated at birth and reared apart in randomly selected homes representing the full range of potential behavior-influencing environments, and meeting each other for the first time when studied, most MZA pairs were only partially reared apart, and grew up in similar cultural and socioeconomic environments at the same time.

In 1990, Peter Neubauer coauthored a book with his son, Alexander entitled: Nature’s Thumbprint. Within the Introduction was a tantalizing hint to what may have been information from Neubauer’s archived study. Father and son said that many years ago an opportunity arose to follow the development of identical twins from infancy. “With great curiosity a number of us decided to study the influence of the environment on the child.” No other identical twin study had explored twins reared apart as they matured and developed from birth on.

We could look at change as it happened. We would be there at birth and continue regular, intensive observations of separated twins and study their relationship with parents and siblings, collecting as much information as possible about behavior and growth. . . . In fact, it was our assumption that only by studying development as it happened could these disorders be accurately understood. Our study would therefore be useful to the investigation of both healthy and pathological growth, as well as to the ways the environment influences that growth.

Twin studies do not provide definitive evidence for the genetic basis of psychiatric conditions. The twin method is no more able to tease apart the potential roles of genetic and environmental influences for behavioral traits and “disorders” than family studies. As Jay Joseph noted, both research methods are confounded by environmental factors and should be evaluated in the same way: “neither provides scientifically acceptable evidence in support of genetic influences on psychiatric disorders and behavioral traits.”

Please note: I reserve the right to delete comments that are snarky, offensive, or off-topic. If in doubt, read My Comments Policy.