4. New ideas - Epigenetics Twin studies Nurture

Why is it if identical twins are genetically the same, that they can have different outcomes in their life, such as one developing cancer or having a different personality. Some recent twin studies seem to point to nurture or environment having an effect on physical and personality traits. The following video explains how one twin developed cancer, while her identical sister did not.

Effects of Environment on Genetics and Behaviour
It is now understood that environmental conditions can actually change our genes, giving different outcomes, and in the case of identical twins, make them very different.  This could cause them to have different diseases or psychological diseases

http://study.com/academy/lesson/behavioral-genetics.html

Environmental Control of Genes
Recently, the study of epigenetics has focused our attention on the negative effects of environment on human genes.  In this case, a pair of identical twins are so different, the effect of something on the genes cannot be denied.