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Separated At Birth. Really.

What happens, 28 years after an accidental maternity-ward switch, when identical sisters meet each other?

A Spanish mother gave birth to twin girls in 1973 at a hospital in Las Palmas, Canary Islands, but one of the babies was accidentally switched in the maternity unit. The mistake went undiscovered for nearly three decades. The mother assumed that she had given birth to fraternal twins (dizygotic, from two eggs) and not "identical" ones (monozygotic, two embryos developed from a single fertilized egg). The girl she named Begoña was her biological daughter; the baby named Beatriz was not.

As Nancy L. Segal relates in "Someone Else's Twin," her fascinating account of the switched-at-birth misstep and the painful family and legal entanglements that followed much later, an unexpected encounter in a clothing store was the tale's turning point. A shop assistant mistook the 28-year-old Begoña for a person she knew named Delia. After Begoña explained who she was, the clerk—still struck by the resemblance—suggested that the two "doubles" meet each other.

It was not the first time that Begoña had been mistaken for someone else; her curiosity was enough to overcome her hesitation to agree to the clerk's suggestion. The ensuing meeting was unsettling. The women discovered that their similarities extended well beyond physical resemblance. "The two of them realized that they push their food away when eating with a fork or spoon and fold their lips over their teeth when they get anxious," Ms. Segal writes. DNA tests years later confirmed what the girls now believed: They were identical twins.

The shop assistant surely never expected that she would set in motion events that would escalate from family drama to legal action against Spanish authorities for the government-operated hospital's error. The families' histories were thrown into disarray. Begoña worried that Beatriz would feel rejected and that their bonds would be severed. Publicity about the case caught the attention of Ms. Segal, an academic psychologist who specializes in twin research and is a twin herself. She flew to the Canary Islands in 2008 to interview the young women and their families.


Someone Else's Twin

By Nancy L. Segal
(Prometheus, 301 pages, $25)

Ms. Segal writes clearly and passionately, and yet she rarely strays from the precise language of a social scientist. The result is an engaging narrative intertwined with a careful attempt to draw sound conclusions from the facts. She discusses some other cases of identical twins being switched at birth (there are only seven known instances) and classical studies of twins reared apart. She knows the terrain well because she was a key contributor to many of the studies.

The book has much to tell us about the ways in which genes, environments and their interactions shape who we are. Of course one should be careful not to extrapolate too much from a single case. It is an extraordinary fact, though, that separated identical twins often exhibit remarkable similarities—down to the smallest gestures—despite not growing up together.

In the switched-baby cases Ms. Segal discusses, it is highly unlikely that the error would have been discovered were it not for the fact that the twins were monozygotic and thus had a striking physical resemblance. But births of monozygotic twins are rare, roughly one in 300. Might there be thousands of solitary children growing up with nonbiological parents because of hospital switches? Ms. Segal thinks that this is indeed the case, and she argues that there is an urgent need to improve the existing rudimentary hospital systems of baby identification.

The reunion of separated twins is inevitably an emotionally exhausting event. In the case of Begoña and Delia, the baby switch went undetected for 28 years. When the truth may be painful, is it better to remain in the dark? Some of Ms. Segal's interviewees say they regret having found out about their long-lost twins, citing the psychological pain of seeing the assumptions of a lifetime vanish. Ms. Segal favors revelation—in cases of adoption, too—because of the medical usefulness of knowing your biological family and the importance of finding out about genetic predispositions to disease. Delia was diagnosed with leukemia in her teens, and it was difficult to find a matching bone-marrow donor. Knowing Begoña would have guaranteed a perfect match.

Today, 38 years after the mix-up in the Canary Islands, the women have arrived at a certain kind of peace. Begoña and Delia live in different areas and tend to keep in touch by phone—Begoña remains wary of hurting Beatriz's feelings by becoming too close to her true twin—but the two share an uncanny understanding. The lawsuit filed by the families produced a judgment in 2009 that awarded them a total of €900,000 (about $1.2 million at the time). Begoña, Beatriz and their mother (her husband had died by then) each received €180,000. Delia, judged to be the most injured—she never met her biological father before he died, and her life was put in peril without the perfect bone-marrow match from her twin—was awarded €360,000. Despite the exactness of such sums, Ms. Segal does not pretend that it is possible to measure with precision the cost of the emotional turmoil endured by the main characters in this thought-provoking drama.

Mr. Cesarini is an economics professor at the Center for Experimental Social Science at New York University.

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