Strange things happen when twins do what anyone else does—get married and have children! When identical twins marry non-twin individuals, the twins each become a “genetic parent” (mother or father) to their twin sister’s or twin brother’s children. That is because identical twins share all their genes and either could have conceived a given child. Similarly, the children for the two families turn into unusually close cousins—actually genetic “half-siblings,” sharing 25% of their genes on average. That is because each child has a parent who is the clone of the other (clones are genetically identical beings).

However, nothing unusual happens when fraternal twins marry non-twins—the ordinary aunt-uncle/nice-nephew relations remain. But comparing these two types of families is a great way for researchers to explore human behavior. I have found that identical twin aunts and uncles feel closer to their nieces and nephews than do fraternal twin aunts and uncles—my research suggests that this is so because they perceive similarities between themselves.

When identical twins marry sisters (or brothers), the children turn into even closer cousins, sharing 37.5% of their genes, on average. In my book, Twin Mythconceptions, I discuss this in relation to the original conjoined twins, Chang and Eng. These twins were physically connected, but able to father over twenty children between them. They would alternate three-day stays at each twin’s home.

I have also been fascinated by people who look alike, but are not genetically related. I came upon a wonderful sample of such pairs in Canada. A photographer, François Brunelle, has an unusual hobby—locating and photographing people who happen to look alike. In my research, I discovered that their personalities and self-esteem are quite different!  This is important for twin research because some critics believe that identical twins are alike because people treat them alike, based on their appearance. But if that were true, then unrelated look-alikes should match in personality to the same degree as identical twins, but they do not. In fact that they do not shows that identical twins evoke or elicit similar responses from people, due to their genetically-based behaviors, rather than the other way around. I am always on the outlook for these interesting pairs.

I am pleased to offer you a look at my new book Twin Mythconceptions with this complementary download of the chapter, “Twin Spouses and Unrelated Look-Alikes: New Views.”

Fill out this brief form for your chapter download of Twin Spouses and Unrelated Look-Alikes: New Views

If you liked this article and the chapter, you can access additional chapters from the book Twin Mythconceptions online via ScienceDirect.

If you prefer to purchase a print or e-copy of the book, visit the Elsevier website. Apply discount code STC317 to save up to 30% off the list price and free global shipping.

Nancy L. Segal received a B.A. in psychology and literature from Boston University (1973), and an M.A. (1974) and Ph.D. (1982) in the Social Sciences and Behavioral Sciences from the University of Chicago. From 1982-1991 she was a post-doctoral fellow and research associate at the University of Minnesota, affiliated with the well-known Minnesota Study of Twins Reared Apart. She is currently Professor of Psychology at CSU Fullerton and Director of the Twin Studies Center, which she founded in 1991. Dr. Segal has authored over 200 scientific articles and book chapters, as well as several books on twins.

You can also read Dr. Segal’s other article here on Rare Twins Born to Parents of Different Ethnicity here and access her educational video on Identical Twins here.