The Truth About Twins

A new book by a prominent twin researcher debunks many popular myths and makes a strong case for how all of us can benefit from studies of identical pairs.

By Brian Boutwell Ph.D., published November 7, 2017 - last reviewed on November 19, 2017

Given that twins are more than 3 percent of the population, it's somewhat surprising that they're the subject of so many misconceptions. Tales of sibling telepathy, mate-swapping, and general eeriness have been with us for centuries. But as research into the origins and life experiences of twins has advanced, more debates can be definitively settled, and it's hard to imagine a better guide to that work than Nancy Segal, director of the Twin Studies Center at California State University, Fullerton, who has contributed to some of the field's foundational studies. Her new book, Twin Mythconceptions, is an entertaining investigation into the scientific basis, or lack thereof, for more than 70 commonly held beliefs about identical and fraternal pairs. These are matters of interest to a growing number of families—the rate of twin births in the United States has risen almost 80 percent since 1980, due primarily to advances in infertility therapies.

Twin Misconceptions by Nancy L. Segal Ph.D.Published by Academic Press 2017

What We Know

The topics Segal tackles range from curiosities to life-or-death questions. Can each member of a fraternal twin pair, for example, have a different father? Not only is it possible, but it's surprisingly common: The most recent study estimated that more than 2 percent of fraternal twin pregnancies involve the genetic contributions of separate men, but Segal contends that it's likely higher, since when the two dads are of the same race and ethnicity, mothers may just assume that both twins have the same sire.

No unusual prenatal events are required for twins to have different birthdays. The longest known interval between deliveries is a stunning 87 days. And twins' birth order matters, but not for the reason you might think: There's little proof, Segal writes, that being delivered first or second has any lasting impact on one's psychological outcome. On the other hand, second-born twins are at a higher risk for health problems including respiratory distress, neonatal trauma, and infections than are their just-older siblings.

Young identical twins often seem to have a telepathic bond, but there's no evidence that it's real. Their similarities reveal something about the likeness of twins' minds, not a link between them. This is one of many findings from research into twins separated at birth and raised apart. When such pairs independently read the same books, follow the same household routines, or enjoy the same hobbies, Segal writes, "they cannot be communicating because they are often unaware that the other twin exists—instead, they are reflecting their matched abilities, tastes, and temperaments."

Another popular notion is that an identical twin could commit a crime and frame the sibling. Not anymore, according to Segal. Modern DNA testing can decisively distinguish the felon from the innocent twin. In general, the idea of good and evil identical twins, and the culture's eagerness to deploy those labels to help tell siblings apart, is inaccurate and potentially damaging. In fact, Segal explains, fraternal twins are more likely than identical pairs to diverge widely in personality because each inherited a different set of genes.

And could one identical twin pretend to be the other and "cheat" with that sibling's romantic partner? Not likely, Segal believes. Even slight differences in facial and physical features, personality, and temperament have been shown to be critical factors in social attraction and mate choice, making it possible for someone to be attracted to one twin but not the other. 

Why We Study Twins

Segal, a fraternal twin, worked on the groundbreaking Minnesota Study of Twins Reared Apart, which found that twins raised separately still share many personality traits, just like twins who grow up together. It was powerful proof of the influence of genes on personality and a vital clue in sorting out genetic and environmental input.

The logic of this work seems simple, but the potential for insight is profound. Studying identical pairs in which one sibling develops a disease while the other does not could swing wide the doors to research on the root causes of conditions affecting millions.

In the end, Segal understands, twins' value as medical research subjects is not what most fascinates us about them. It's something more philosophical: "The idea that physical and behavioral traits can be closely replicated in two infants, children, or adults runs counter to our expectation that no other person in the world could be like us."

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