Which of us has not wondered how our life might have turned out had certain circumstances been different? Like characters inhabiting parallel universes in science fiction plots, we all live only a tiny fraction of the lives that might have been.
As a teenager I won an acting award, spent two years learning the Russian language (which I have hardly spoken since), and avidly pursued astrophotography (my first publication, at the age of fourteen, was a photograph of an aurora borealis that appeared in Sky and Telescope). Each of these early interests somehow faded and was overtaken by others that ended up having a more lasting impact on my life. Yet had a teacher or opportunity encouraged me to concentrate on any one of these early preoccupations, I suspect that my life, and perhaps even some aspects of my personality, might be considerably different today.
In a reflection on the mysteries of human development, the behavioral geneticist Robert Plomin and the psychologist Denise Daniels once entertained a similar question about the life of Charles Darwin.1 When Darwin was invited to sail as a naturalist on the voyage of HMS Beagle, his hopes, and perhaps with them the revolution that bears his name, were almost dashed. First, Darwin’s father objected to the voyage as a “useless undertaking” that would divert his son from an intended career in the clergy. It was only through the strenuous intervention of Josiah Wedgwood, Darwin’s enlightened uncle, who saw in young Darwin “a man of enlarged curiosity” and who drove thirty miles to confront his father, that Robert Darwin’s objections were finally overcome.2
Then the captain of the Beagle—twenty-six-year-old Robert FitzRoy—balked at the selection of young Darwin. An ardent disciple of Johann Kaspar Lavater’s theories about physiognomy and its relation to character and personality, FitzRoy was convinced that the shape of Darwin’s nose indicated a lack of sufficient energy and determination for such an undertaking. Darwin eventually succeeded in winning FitzRoy over, and FitzRoy himself later came to the conclusion that Darwin’s nose “had spoken falsely.”3 How might the history of science have differed had Darwin not circumnavigated the globe on HMS Beagle, visited the Galápagos Islands in 1835, and developed his earliest ideas about evolution based on his five-week visit to this veritable laboratory of evolution in action? We can only speculate about such counterfactual historical possibilities.
Nancy Segal’s Indivisible by Two makes use of a particularly powerful research method for answering such vexing questions about why our own and other people’s lives turn out the way they do. Segal studies twins—identical, that is, from a single fertilized egg, and fraternal, from two eggs fertilized by different sperm—as well as pseudotwins, children of the same age who are raised together. She does so with a passion that derives in part from the fact that she is a fraternal twin herself. A Distinguished Professor in Humanities and Social Sciences and director of the Twin Studies Center at California State University, Fullerton,…
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