Daphne Goodship and Barbara Herbert [were] identical twins adopted into separate British families after their Finnish mother reportedly killed herself. They reunited, aged 40, in 1979. Unlike their adoptive families, they were both incessant gigglers, had a fear of heights, dyed their hair auburn, and met their husbands at town hall Christmas dances.
Cases such as these have been used to revive the notion that distinct upbringings make no difference in how we turn out: it’s all down to biology, specifically the clockwork mechanisms of Mendelian genetics – an idea with a long historical tail. But much has changed in our understanding of genetics since the human genome was sequenced in 2003.
Yet many of those involved in twin studies have been resistant to these influences, betraying the influence of a deeply rooted magical thinking around twins that has cast its long shadow over our understanding of the line between selfhood and otherness.
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Twin studies are still widely used and may remain useful in trying to find out the heritability of illnesses and other physical outcomes where the environmental component is unlikely to differ between identical and fraternal twins. But there is a huge gap between attaching a heritability percentage for, say, macular degeneration, and for something like IQ or academic performance, where it’s impossible to untangle the interlocking influences of biology and culture.
