Identical twins separated at birth tend to have more similar personalities than identical twins who grew up together. The hypothesis is that, due to the presenc

Identical twins separated at birth tend to have more similar personalities than identical twins who grew up together. The hypothesis is that, due to the presence of each other when growing up, they actively (though subconsciously) differentiate themselves. It's reasonable to assume the same effect would happen to some extent with other siblings. The fact that twins (or siblings) are more similar when raised in different environments would seem to contradict the idea that subtly different environments are responsible for the majority of sibling discrepancy. I'm working on finding the citation. The phenomenon is known as "twinning" but that apparently refers to a bunch of other junk that makes it hard to find what I'm referring to. I may have to dig out my bioethics book when I get home.