Gene-environment correlations can arise by both casual and non-casual mechanisms. Genetic variants influence environmental exposure indirectly via behavior. Nearly all diseases result from a complex interaction between an individual’s genetic make-up and the environmental agents. Subtle differences in genetic factors cause people to respond differently when exposed to the same environmental agent. As a result, some possess a low risk for developing a disease through an environmental insult, while others are much more vulnerable. It is common knowledge that some environmental exposures, such as x-rays or toxic chemicals, can directly damage genetic material. It is more difficult to imagine how environmental factors such as a nurturing parent or traumatic experience can have an effect on a DNA molecule.