Murder Gene
This is not a scientific or forensic expert term per se but is a term made popular by the media to refer to particular variant of the monoamine oxidase A (MAOA) gene which apparently predisposes an individual to disproportionate violence under any condition that triggers the carrier of that gene sequence to violence.

As referred to in Mobley v State, "a possible genetic basis for violent and impulsive behavior in certain individuals."
Also known as the rage gene and even more commonly, the warrior gene because of the theory that persons with this gene would likely have been the most efficient: berserk warriors on a field of battle.

The murder gene, the theory goes, suggests that behavior and activities from infancy into childhood, and through adulthood, were not the products of free will as society defines this term because the individual lacked the ability to make non-impulsive, considered choices about his life's path.1 Authors Ishikawa and Raine suggest, in their 2002 article, that though then viewed by the courts as exotic evidence:

"There are simply too many studies, in too many countries, using different methodologies that converge on the same conclusion: genes do play a role. Second, other, potentially less controversial fields of behavioral trait research have not only identified heritability in psychiatric disorders such as autism, schizophrenia and reading disability, but also in personality traits such as political conservatism.

"Thus, it would be surprising if criminal behavior - particularly recidivistic crime - was not in some way influenced by genetic factors."