What can twins raised apart really tell us about nature and nurture?

Twins let scientists separate genes from upbringing, up to a point. Identical twins share essentially all their genes; fraternal twins share only about 50%, like any siblings. If identical twins are considerably more similar than fraternal ones, this implies that genes play an important role in the trait.

Twin researchers split the differences between people into three parts: additive genetics (A), common environment (C) and unique environment (E), the ACE model. The cases that make headlines are twins raised apart. Jim Springer and Jim Lewis were separated from birth and reunited at age 39; reports said both had a dog named Toy and named their first sons almost identically. Coincidences like these are anecdotes, not evidence, and their researcher, Thomas Bouchard, stressed that the Jim twins happened to be unusually alike, while most twins show more differences, and that identical twins raised apart are on average about 50 percent similar.

The method rests on the equal environments assumption: that identical and fraternal twins share equal family environments. It can be tested where parents wrongly believed identical twins to be fraternal, and the twins stayed as alike as identical twins raised as identical. But researchers continue to debate whether it is valid, and critics argue that twin studies rest on questionable assumptions.

Heritability can also differ by circumstance. Eric Turkheimer and colleagues found in 2003 that the environment accounted for about 60% of the variance in IQ among low-income children while genes accounted for almost none of it, and found the reverse for wealthy children. Later studies were more mixed: a 2016 meta-analysis found the effect in US studies but not, or reversed, in Australia and Western Europe.