According to a new study published in the journal Nature Genetics, identical twins may not be as 'identical' as previously assumed. The study states that genetic differences start early on in embryonic development. This revelation could have implications on the effects of nature versus nature. Environmental factors have been presumed to be the reason behind physical or behavioral differences but the new study attributes genetic factors to the differences. It concluded that identical twins differ by an average of 5.2 early developmental mutations.