Twins are often used in studies designed to uncover the effects of nature and nurture. Identical twins share the exact same genes, so some researchers hypothesize that differences between them must be due to the environment. By comparing the results between fraternal twins and identical twins, researchers can begin to determine the relative extent that nature or nurture played in any given outcome. The environment has a key role in shaping our epigenetic profiles, which in turn influence the activity of our genes, which in turn may shape our behaviour, choices and health – our environment – and so it goes on. That might explain why the epigenomes of identical twins diverge over the years, as a 2012 study showed. However, even when identical twins are brought up together, only about 50 percent of their traits are alike. The twins have the same genes, the same family environment and the same exposures to peers but still have differences. Today, more than 1.5 million twins around the world take part in studies aiming to assess the relative roles of genes and the environment in everything from ageing to disease, and from bullying to religious belief.