When most people hear “environment,” they automatically think of the social environment, such as how children are raised or what kind of interpersona… - J. Michael Bailey

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When most people hear “environment,” they automatically think of the social environment, such as how children are raised or what kind of interpersonal experiences they have. But to a geneticist “environment” means anything not encoded in DNA, and this includes biological factors as well as social factors: diet, germs, and random biological events might also affect development. For example, when one identical twin is born with a major brain defect such as anencephaly (lack of a cerebral cortex, or outer brain) or microcephaly (a very small cortex), the other twin is usually normal. Only environment can cause identical twins to differ, but the environmental problem in this case is biological.