Twins, especially if they are identical, tend to receive much attention from people, both in terms of comments about their physical resemblance, but also when it comes to similar behavior. Born Together --Reared Apart: The Landmark Minnesota Twin Study by Nancy L. Segal, describes the MISTRA program (the largest one of its kind) where reared apart monozygotic (identical twins) and dizygotic (fraternal twins) were studied to measure and further understand and advance genetic influences and genetic studies. As such, even though the books purpose might not be to weigh in on the often termed "nature v nurture" discussion as much as it generally describes the MISTRA study, it still sheds light on this controversial issue.
Twin studies might not just be of interest to the general public. In the beginning of the book, Segal acknowledges that her interest in twin studies is in part because she is a fraternal twin. Similarly, I find twin studies fascinating at times and I am an identical twin. Twin studies come in many forms, and my sister and I also participated in a (reared together) twin study in Sweden when we were younger where our mother and the two of us filled out many long surveys about out behavior. The MISTRA study is however very significant because of the large number of reared apart twins (both MZ and DZ). When researching genetic factors, twins being reared apart are the best sample, because most environmental similarities, based on being reared together can be discounted as well as compared to twins reared together.
When discussing identical twins, Segal uses the term MZ, which stands for monozygotic, and MZA for monozygotic reared apart. The terminology used for fraternal twins is DZ, which stands for dizygotic, and DZA, which is dizygotic, reared apart. For twins reared together, the abbreviation MZT and DZT is used. If not familiar with the terminology, it may be difficult to remember all the information presented concerning the abbreviated distinctions between the MZA, MZT, DZA and DZT. Even though the study of genetics, the statistics used and the correlations described may be difficult to understand, Segal does make it easier, through her writing, for the reader to connect with the findings, the hypotheses and the research. The book also contains numerous pictures of MZA and DZA twins that are used to explain such findings. As such the intended audience of the book is perhaps foremost researchers and those either interested in or working in the field of behavioral genetics. At the same time, with some patience, anyone interested in twin studies will find the book interesting.
Segal describes and explains how the MISTRA study concluded that genetic factors are more important than environmental ones. According to Segal, "MZ twins inherit 100 percent of the same genes and, consequently, all their unusual gene combinations, whereas DZ twins inherit 50 percent of the same genes, on average, by descent" (p. 58). DZ twins are therefore no more alike than full siblings, whereas MZ twins are more or less "clones". The fact that MZ twins are usually so very much alike is therefore not difficult to comprehend, since they share the same genes. Segal describes how MZ twins, whether reared apart or together showed greater correlation of similarity than DZ twins (reared together or apart). When Segal states that "the striking findings from so many of our analyses was the negligible effect of the rearing-home environment on behavior" (p. 132-133), I would like to add to the end of that sentence, in monozygotic twins. The situation of MZ twins is exceptional in the sense that they share 100 percent of genes. Therefore, adding that nature has a stronger influence on nurture, while not including that this is true for MZ twins more so than DZ twins or simply siblings, may be slightly misleading, especially when paired with the later statement; "About half the variation in most behavior is explained by genetic factors, although some traits such as brain waves show greater genetic influence, and other traits such as job satisfaction show less" (p. 326).
Basically, for fraternal twins and full siblings both "nature" and "nurture" seems to play important roles, but when it comes to identical twins, "nature" (genes) plays an even more significant role. This is understandable since identical twins share the exact same genes.
As Segal states, "The bitter 1970s nature-nurture debates have largely (but not completely) subsided as evidence of genetic influence on behavior has accumulated" (p. 298), it does seems as if we are more comfortable and accepting when discussing the influences of both nature and nurture than we were before, even though there are still many researchers and academics that feel strongly about the situation (on both sides). At the same time, the discussion of genetics and heritability has become widely used in popular culture to explain gender differences, often to the advantage of men, but without much substance to back these claims up. As such popular culture has used research in other fields, such as genetics and human behavior and applied those loosely to explain (and exaggerate) gender differences.
© 2013 Hennie Weiss
Hennie Weiss has a Master's degree in Sociology from California State University, Sacramento. Her academic interests include women's studies, gender, sexuality and feminism.