Hello, I’m Wang Fang. In 1993 geneticists studied men in a Dutch family with numerous violent offenders, including arsonists, rapists, and murderers. Every man carried a low-activity monoamine oxidase A gene on the X chromosome (Brunner et al., 1993). Monoamine oxidase A, or MAO-A, is an enzyme regulated by the MAOA gene that breaks down neurotransmitters such as dopamine, norepinephrine, and serotonin.

Low activity of this gene produces less enzyme to break down those neurotransmitters, leaving higher concentrations in the body. High concentrations are known to associate with anger, aggression, and antisocial behavior. The MAOA gene consequently acquired the namethe violence, warrior, or warrior-gene variant.

On October 16, 2006, a Tennessee man brutally slashed his wife and shot her friend dead. Prosecutors charged him with first-degree murder, but his lawyer argued that he carried the MAOA variant. A person cannot choose their genetic makeup or control gene expression, so he should not bear full responsibility and at least should not die for the crime.

If you were the judge or a juror,how would you decide when faced with this genetic defense? If people cannot be responsible for genes that make them more prone to violence, are they still responsible for their acts? Further, if some are born with dangerous genes, should society test and monitor them?

Understanding these questions affects not only how we explain behavior but related laws and social policy. More importantly, the issue extends beyond violent crime to debates about intelligence, personality, sexual orientation, sex differences, and more.For instance, how much behavioral difference between women and men is biological and how much cultural? We will discuss this later.

Where should we begin? With what we inherit from our parents—