From Slate:

Life Is Random Biologists now realize that “nature vs. nurture” misses the importance of noise. By Cailin O’Connor Is our behavior determined by genetics, or are we products of our environments? What matters more for the development of living things—internal factors or external ones? Biologists have been hotly debating these questions since shortly after the publication of Darwin’s theory of evolution by natural selection. Charles Darwin’s half-cousin Francis Galton was the first to try to understand this interplay between “nature and nurture” (a phrase he coined) by studying the development of twins. But are nature and nurture the whole story? It seems not. Even identical twins brought up in similar environments won’t really be identical. They won’t have the same fingerprints. They’ll have different freckles and moles. Even complex traits such as intelligence and mental illness often vary between identical twins. Of course, some of this variation is due to environmental factors. Even when identical twins are raised together, there are thousands of tiny differences in their developmental environments, from their position in the uterus to preschool teachers to junior prom dates. But there is more to the story. There is a third factor, crucial to development and behavior, that biologists overlooked until just the past few decades: random noise.

Sure, but the existence of random noise in development means that twin studies actually underestimate the importance of Nature relative to Nurture. Some of the differences between identical twins, including identical twins raised apart, are not caused by Nurture but by random noise. But because the default assumption is that differences between identical twins are caused by Nurture, the existence of random noise means that Nurture is overrated in importance.

Besides the third factor of noise, there are also fourth and fifth factors: sibling rivalry and the need for role players. These also tend to exaggerate the role of Nurture in twin studies.

For example, I know two identical twins who have meticulously documented their differences. One told me that his eyesight was 20-22 while his brother’s was only 20-24. At present, one twin wants to be an actor while the other wants to be an engineer. If they were raised apart they might well be be more similar.

It’s not uncommon for twins to insist they are fraternal, not identical, out of the narcissism of small differences: the Hamm gymnasts were convinced they weren’t identical because their hair swirls in different directions and the Olsen twins insisted they are fraternal.

Moreover, identical twins are often pulled apart by society’s need for them to play different roles. Consider the former NBA players Jason (7′-0″) and Jarron (6′-11″) Collins. They are both natural centers, but teams don’t need two centers. Playing together through college, the slightly better Jason was the center while Jarron was relegated to power forward. If they had grown up separately, both would have likely played center on whatever amateur teams they were on.

The Grant brothers of the 1990s NBA grew up playing together so Harvey was the shooting forward while Horace was relegated to being a skinny power forward. Interestingly, Horace had a better NBA career, perhaps because he had to work harder as a youth.

Here’s a Taki’s article I wrote back in 2010 on identical twins.