Because identical twins are almost genetically identical, they make for an interesting test group when assessing differences in lifestyle over a prolonged period of time. If you mix in the different factors, say for instance smoking, you can get a precise picture of the effects of smoking or not on a closely identical genetic sample. But in this case, you don’t even need fancy tests to see the difference in twins that smoke and their counterparts who don’t smoke. The difference is blatantly clear.