Twins Reared Together and Apart: The Science Behind the Fascination

By Segal, Nancy L. | Proceedings of the American Philosophical Society, March 2017 | Go to article overview

Twins Reared Together and Apart: The Science Behind the Fascination


Segal, Nancy L., Proceedings of the American Philosophical Society


The reasons behind the widespread interest in twins are fascinating to consider. There is no question that people are intrigued by twins-wherever I go I am asked about my work by both professionals and non-professionals. I often ask them, "Why do you care about this topic?" Most of them reply, "Well, it is just so interesting," but no one can provide a more substantive answer. I have thought a great deal about this question. I believe that twins are fascinating because we expect and anticipate individual differences in behavior and appearance. Therefore, when we encounter two people who look so much alike and act so much alike, it becomes intriguing because it challenges our beliefs in the way the world works. For some people, this may be disturbing, but for many, it is quite compelling.

There are also many misunderstandings surrounding the origins of twinning and twin development. I address some of them in this paper and many others in my new book Twin Mythconceptions (Segal, 2017).

Two Twin Types

The study of twins rests on the availability of two types of twins. Identical, or monozygotic (MZ), twins share all of their genes in common, having split from a single fertilized egg between the first and fourteenth post-conceptional day (Segal, 2000). Fraternal, or dizygotic (DZ), twins result from the fertilization of two separate eggs by two separate sperm and share 50% of their genes, on average, by descent, just like full siblings. Fraternal twins come in two varieties, same-sex and opposite-sex pairs. These twins are quite intriguing, even though they lack the visual interest of identical twins, because the different pairs lie on a spectrum of genetic relatedness. That is to say, some pairs share higher proportions of genes related to certain traits than others. Note that the same-sex twins in Figure 1 look quite different, whereas those who are opposite-sex look quite alike.

The logic of the twin design is simple and elegant. One simply compares the degree of similarity between identical twins with the degree of similarity between fraternal twins with respect to a given trait, such as height (Segal, 2012). For most measured characteristics, we find that identical twins are more alike than fraternal twins. This finding provides evidence of a genetic component underlying the trait under study. Height is a trait that is strongly influenced by genetic factors (correlations are 0.86 for reared-apart MZ twins and 0.93 for reared-together MZ twins; Bouchard et al., 1990). This finding is exemplified by two young women who the Guinness World Records 2007 has acknowledged as the tallest identical female twins in the world, at 6'8". The twins were volleyball players during their college days in Florida, and I appeared with them on the set of ABC's Good Morning America.

I now want to turn attention to the famous identical McKusick twins, Victor and Vincent, who were born on a farm in Parkman, Maine, on October 21, 1921 (Segal, 2016). The McKusicks are emblematic of twins who are both distinguished and illustrious-both of them were members of the American Philosophical Society. Before providing an overview of their lives, I would like to acknowledge the generosity of the twins' families-when I first told them about this talk, Vincent's daughter Kay Ralston sent me several photographs of the twins at different ages. Examining their pictures across different ages is an informative exercise because it shows that physical development is largely under genetic control across the lifespan.

In Figure 2, the twins are shown at about age 8 and appear identical in height. We see them again at about age 10 in Figure 3. A great deal of physical development takes place between the ages of 8 and 10, yet the twins are virtually indistinguishable at that age. Finally, let's fast forward to the age of 80-even after 70 years of development it is clear that these twins are still virtually indistinguishable (Figure 4).

The remarkable thing about twins is that they tell us so much about human behavioral and physical development just by being themselves. …

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