Sociobiology was extremely controversial in its early days in the 1970s and 1980s. To say that a behavior had a genetic basis seemed to imply that it was morally desirable or inevitable. Further, the emphasis on genetics was seen to invalidate the importance of environment. We now know that environment and genetics interact in almost all of human behavior. While genetics may set the outer limits of behavior, environment has huge influence on the expression of behavior and even on the expression of genes themselves. Genes can be turned on and off according to environmental influences. The biggest problem with evolutionary explanations of human behavior, however, involves the profound difficulty distinguishing between proximate and ultimate levels of causation.