For Twins, Nature (Finally) Trumps Nurture

The “nature/nurture” debate has raged among developmental psychologists for decades. For most of human history, nature––that is, genes and heredity––was held to be the primary determinant of one’s tastes, capabilities, and personality. But looking to genetic origins to explain an individual’s development eventually fell out of favor. By the mid-twentieth century, psychologists and educators had shifted predominantly to the nurture side of the developmental spectrum, upholding that all individuals have the same potential at birth, regardless of social class, race, sex, or minor biological traits. This fueled the utopian project of public education, especially higher education, which grew to an enormous scale in the United States. Under this model, individual development is highly plastic and can and must be shaped by one’s environment at an early age to ensure proper development. But one team of researchers at the University of Minnesota has shown the cuddly cult of nurture to be largely unfounded by studying twins separated at birth. Their 30-year project is documented in lead researcher Nancy Segal’s new book, Born Together––Reared Apart: The Landmark Minnesota Twin Study, published this month by Harvard University Press [1]. One reviewer writes:

In early 1979, a pair of identical twin brothers who had been separated at four weeks were reunited after 39 years. Both named Jim, they discovered that they smoked the same brand of cigarettes, vacationed in the same town and both called their dog “Toy.” Struck by the story, psychologists at the University of Minnesota started studying separated twins that same year. Their efforts blossomed into the Minnesota Study of Twins Reared Apart, which ran for a quarter century, attracting world-wide fascination and antipathy.

Nancy Segal’s “Born Together—Reared Apart” is a thorough history of the project and of the 137 pairs of star-crossed twins who made it possible. Ms. Segal, a key member of the Minnesota team, focuses on the many scientific publications that emerged from the data. But along the way, readers meet leading twin researchers and a whole lot of twins—including the “Jim twins,” the “giggle twins” (who both laughed almost nonstop) and, most incredibly, Oskar, raised as a Nazi, and his identical brother, Jack, who was raised as a Jew…

[T]he logic of the Minnesota study is simple. When identical twins are raised apart, you can disentangle nature and nurture for a given characteristic by simply measuring how similar the twins are. You can double-check your answer by comparing the similarity of identical twins (who share all their genes) and of fraternal twins (who share only half their genes).

The Minnesota researchers tracked down every pair they could find—and measured traits related to almost every aspect of life: health, cognition, personality, happiness, career, creativity, politics, religion, sex and much more. The Minnesota study reveals genetic effects on virtually every trait. The breakdown between nature, nurture and everything else varies from trait to trait. But Ms. Segal emphasizes the uniformity of the results—the consistent power of genes, the limited influence of parenting. [2]

Opposing what many have called the nurture camp’s “meritocracy myth,” Segal illustrates how, according to her team’s thirty years of research, nature has a far greater impact on development than nurture, so individuals don’t actually earn, or freely choose, their strengths and weaknesses. Rather, they are genetically ingrained and not easily shaped. The book’s publisher describes the huge implications of Segal’s work for parenting, education, and social policy:

As a postdoctoral fellow and then as assistant director of the Minnesota Study, Nancy L. Segal provides an eagerly anticipated overview of its scientific contributions and their effect on public consciousness. The study’s evidence of genetic influence on individual differences in traits such as personality (50%) and intelligence (70%) overturned conventional ideas about parenting and teaching. Treating children differently and nurturing their inherent talents suddenly seemed to be a fairer approach than treating them all the same. Findings of genetic influence on physiological characteristics such as cardiac and immunologic function have led to more targeted approaches to disease prevention and treatment. And indications of a stronger genetic influence on male than female homosexuality have furthered debate regarding sexual orientation. [3]

Segal’s book recounts decades of groundbreaking research, which, though contested by many psychologists, has had a ripple effect across the nature/nurture developmental spectrum. If the Minnesota research team is right, your rearing matters less than you might have thought, and your favorite band or flavor of ice cream is more the result of random genetic recombination than upbringing or environmental conditioning, much less free choice.


[1] Nancy L. Segal. Born Together––Reared Apart: The Landmark Minnesota Twin Study. Cambridge, MA: Harvard University Press, 2012.
[2] O Brother, Who Art Thou? The Wall Street Journal
[3] Book jacket for Born Together from the Harvard University Press

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