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Many twins and their families are uncertain about what type of twins they are or have – identical or fraternal. Recently, I have had some rather fascinating experiences with twins along these lines. I was contacted by one member from a 54-year-old female twin pair—I will call her Sally. Sally and her twin sister, Susan, had always believed that they were fraternal twins. This is what they had always been told; after all, the delivering physician had confirmed the presence of two sacs.
However, about eight years ago, someone asked Sally why she was so sure that she was a fraternal twin. This person, an identical twin himself, was part of an opposite-handed pair and he noted that Sally was, as well. (Note that approximately 25 percent of identical twins show such mirror-imaging effects, often due to the delayed splitting of the fertilized egg. At the same time, fraternal twins can also differ in hand preference, associated with inheriting different sets of genes from the parents.
Therefore, differing in this trait is not conclusive with respect to twin type, or what scientists term zygosity.) The twins' birthday is in February 2008, and Susan gave Sally a DNA test kit as a gift. The twins had considered DNA testing prior to this, but the cost was prohibitive; now testing can be done through the mail with the help of many genetic laboratories for about $125.00 per pair, simply by providing a saliva sample. The twins sent in their sample, waited several weeks, then called the lab.
Susan was calm, while Sally paced back and forth. She was certain that she and Susan were fraternal twins, being fraternal felt normal, but she worried that they might be identical because then she could not tell her mother who was deceased. This would make her feel “privy to information that was not mine.” The news: The twins were identical. Sally looks at Susan differently now. She claims that she has never felt closer to her sister, despite their lifestyle differences (one twin is married and the other travels the world.) “I did not think it was possible to feel any closer—because we were so close before.”
Susan and Sally reminded me of another twin pair, my high school friends, Barbara and Judy. I sat next to them in many classes because our last names were close alphabetically. I always believed that they were identical twins, yet they had also been told that they were fraternal and stuck to that. But they were very similar behaviorally and physically, and very close to one another. I lost touch with them for many years, but we found each other again through various contacts. I mention their wonderful episode in Indivisible by Two: Lives of Extraordinary Twins. They noticed some unusual similarities in their children (who would be equivalent to half-siblings if the twins were identical; I will write more about that in a later post) and began to wonder if they might not be identical. I arranged to have their DNA tested, and my high school suspicions were confirmed. Upon learning that she was identical, Barbara wrote, “It’s a nice profound feeling.”
Research shows that identical twins are socially close than fraternal twins, on average. It may be their similarities in intelligence, personality, and temperament and, or, their perceptions of their similarities that drive their relationships. In both these cases, the twins believed for a good part of their lives that they were fraternal—yet they acted otherwise.
I want to describe another revealing pair that I recently studied as part of an ongoing project in my lab at Cal State Fullerton—a study of behavioral similarities, differences, and adjustment in young Chinese twins adopted apart, young Chinese twins adopted together, and young virtual twins (same-age unrelated siblings reared together). One parent was certain that her twins were fraternal, yet my scoring scheme did not yield a definitive answer. Inspecting the girl’s pictures suggested that they were identical, but the poses were slightly different and one girl was wearing glasses. I suggested having their DNA tested and the family agreed. Several weeks later the results came back, showing that the twins were, in fact, identical. This bit of information was surprising to their mother, but it made sense to me in light of the data I had gathered on them.
I tell parents that they are not good judges of twin type because they are sensitive to subtle differences between the twins. I, on the other hand, meeting twins for the first time or looking at their photographs, am usually a better judge. It is strange, but true, that someone who does not know the twins can know so much about them. I am also asked why it may be important to know one’s twin type—because I believe that it is. In the event that one twin develops an illness or a disorder, it would be important to know the chance that the other twin might show the same symptoms. Being identical or fraternal might mean a lot in such cases—although while identical twins show greater match than fraternal twins on most measures, they do not show perfect similarity.
In fact, recent work has shown that even identical twins can show DNA differences, in what are called copy number variations (when a gene exists in more than one copy). Studying such twins promises to tell us a great deal about the roots of normal and abnormal behaviors.
In closing, I will mention one more curious case that bears on the issue of twin type. My study of Chinese twins reared apart requires DNA testing to confirm that the children are identical or fraternal twins. Note that many of these twins are reunited because they look alike. However, a problem for the non-identical twins is that without biological parent information, it is not possible to know for sure that the two children are related; it is only possible to assign a full-sib or half-sib index. I had the pleasure of working with the families of two little girls who bore a striking resemblance to one another—yet the lab indicated that they were not related at all. Disappointment and regret followed, testimony to the special qualities of being or having twins.
