Twin Study: The Two Jims


According to Nancy L. Segal’s new book Born Together—Reared Apart: The Landmark Minnesota Twin Study, the starting point and inspiration for the famous twin studies conducted by her and her colleagues were two identical twins who had been separately adopted shortly after their birth.  It was 1979, the two men were 39 years old, and they had just made their first contact with each other since they had been separated (although they had known of each other’s existence since childhood).

There were numerous resemblances between the two men that struck everyone, from the press and public to the researchers at the University of Minnesota.  There were of course the physical similarities, not surprising in identical twins.  They were similar in weight and height, and naturally their faces were so alike that one could easily be mistaken for the other.

Their medical profiles were also similar.  Both had developed heart disease earlier than normal, although one had already had two heart attacks while the other had been hospitalized once for a less serious cardiac problem.  In terms of posture and other anatomically-influenced characteristics, again they were similar.

There were also a number of behavioral and personality similarities.  They had similar smoking habits, liked carpentry, got stress headaches, and scored closely on objective tests of personality traits and intelligence.  It is not surprising, therefore, that the researchers wondered to what extent these similarities were determined by their shared genes.  Why would two men raised apart and not meeting until they were 39 be so similar, if learning or culture are the primary determinants of human behavior?  The research project expanded, many more twin sets, both reared together and reared apart, were analyzed, and the conclusion was reached that personality is 50% genetic and intelligence is 70% genetic.

Perhaps.

In order to reach such a conclusion, factors other than the observed similarities between the twins would have to be considered.  We can assume that, given that they are monozygotic twins, their genes are exactly the same.  But the researchers seem also to assume that, because they were reared apart, their environments were not the same.  And in terms of particular details, this might be true.  But an assumption is not evidence; the degree of environmental difference would have to be examined in order to factor those differences into the study.

Little information is provided on the environmental backgrounds of the two men, but there are some hints.  One twin was reared in Lima, Ohio, the other in Piqua—two modestly sized towns separated by a mere 40 miles.  They were born in 1940, so during their lifetime Ohio went through an industrial boom-and-bust which both men would have experienced.  The industrial economy of Ohio might explain such things as their shared interest in carpentry, rather than say classical music.  It could also explain their shared preference for blue Chevrolets, a model that has definite social class associations.  That their smoking habits were similar could as easily be explained by social factors as genetic:  smoking was commonplace for their generation, particularly for men, and given both the limited numbers of brands, the impact of advertising, and the social and class associations of particular brands, similarity of smoking habits would not be all that surprising.

That two middle-aged men reared in moderate-sized towns in 1940’s and 1950’s Ohio would share many characteristics in common, twins or not, should also not be surprising.  It would be even less so if the adoptive families were also culturally similar, if they belonged to the same social class, had similar incomes and occupations, attended similar churches, and had similar educational attainments.  Culture gives us many choices in both behavior and preferences, but it also imposes constraints—of language (including colloquialisms), educational and occupational opportunities, beliefs, food habits, style of dress and hair, political leanings, and so forth.  For example, it would not be surprising to find that two people living in the suburbs of Minneapolis/St. Paul would have chosen to purchase a split-level ranch-style house, as such a house is one of the most common layouts for suburbs in that area.  (My brother and his wife as well as their daughter and her husband both live in a suburb of St. Paul and in houses almost identical in floor plan, even though the daughter is adopted and not biologically related to her parents—in fact, she is Korean.  Just as coincidentally, but explainable in terms of culture rather than genetics, my younger brother lives in a house of identical floor plan—in Colorado Springs!  The ubiquity of the architectural style explains the similarities just as well, if not better than, genetics.)

One shared trait has struck everyone who has read the story of these two men, the fact that they are both named “Jim.”  A striking coincidence indeed, but clearly not one determined by their genes; their names were chosen by their adoptive parents, not by the boys themselves, and this fact underscores the importance of culture rather than genes.  It suggests that there was much more similarity between the two adoptive families than has been indicated by the reports, and that the researchers did not pick up on this likelihood and explore the social context in which the two boys were raised suggests the possibility of a major flaw in their approach; it may even suggest a prejudice (in the sense of a pre-judgment) towards genetic explanations of personality and behavior that prevented them from examining the social context and biased their findings.

In the ongoing tussle between nature and nurture (which I call the naturist and nurturist views), nature is often seen as deterministic while nurture is seen as non-deterministic.  It is the determinism of nature that most often riles nurturist opposition, and it is the potential for manipulation of nurture that most often riles the naturists.  Naturism is often seen as fascist while nurturism is often seen as communist.  Nazi race theory vs. Leninist class theory.  Extremes of either side are probably both wrong.  We would not exist as physical human beings without the genes that determine our biological development, and we would not exist as recognizable people without the culture that shapes our lives and provides the opportunities (as well as the constraints) of our particular experiences and ways of life.  Genetics may determine 50% of our personalities—but culture determines the other 50%; and even if genes determine 70% of our intelligence (IQ), culture provides the other 30% (nearly one third) as well as the possibilities for using and expressing that intelligence (as Malcolm Gladwell shows so well in his book Outliers:  The Story of Success).

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