If we take a baby human and a baby monkey and give them both the best environment that we can imagine, the child will be radically different from the monkey and the differences will be almost totally caused by genetic differences. At the extreme, the proponents of the Nature side of the controversy would have us believe that the differences in human behaviour are like the difference between human behaviour and monkey behaviour, just less extremely.
Similarly, if we take identical twins, and give one the best environment possible, and put the other one in closet for eighteen years the differences will be profound, and caused totally by environmental differences between the two children. At the extreme the proponents of the Nurture side of the controversy would have us believe that the differences in human behaviour are like the differences in the twin's behaviour.
Believers in the Nature side of the argument tell you that split-twin studies are wrought with misinformation and bias. Twins who volunteer for that sort of research are perhaps more likely to have kept in touch, and thus, they’ll be more similar to each other due to their continual exposure to each other. A problem arises in the opposing data: qualities like intelligence and sociability are delicate and hard to measure quantifiably. If only fractions of identical twins with identical "natures" exhibit some behaviours (such as homosexuality or even sneezing