"My genes made me do it"

Psychiatric News November 3, 2006Volume 41, Number 21, page 12© 2006 American Psychiatric Association

Jury Still Out on Impact Of Genes on Trial Verdicts

Mark Moran

Americans should not be surprised to hear that claim made by criminal defendants as the genetics of behavior, especially antisocial behavior, are explored by science and popularized.

Paul Appelbaum, M.D., chair of APA's Council on Psychiatry and Law, told psychiatrists at APA's 58th Institute on Psychiatric Services last month that the findings of behavioral genetics—even such preliminary findings as have been made to date— are making their way into the American legal system.

He predicted, however, that genetic arguments are not likely to be successful in freeing defendants from guilt for their crimes, but may more likely be advanced in criminal cases as mitigating factors that should be taken into account in sentencing. Yet even there it remains to be seen how a genetic propensity will be viewed by juries and judges; such evidence could just as conceivably be seized upon as an argument against a defendant, Appelbaum said.

"If effective treatment becomes available, the pressure to identify [at-risk individuals] through screening at birth may be irresistible."

Still, the groundwork for the logic of a genetic defense, in the form of the insanity defense, has already been laid by centuries of case law.

"Anglo-American law has created categories to excuse defendants from culpability when their capacity to choose their behavior is significantly impaired," Appelbaum said. "If mental disorders that impair appreciation of wrongfulness or ability to control behavior negate culpability, why shouldn't genetic determinants have the same effect?

"Why should there not be a defense of genetic determinism, a `my genes made me do it' defense? The logic [of moving] from the existing insanity defense to such an argument is not so absurd that it has not already begun to make an appearance in our courts."

Linking MAOA and Violence

Already rippling through the legal system with intriguing implications is a landmark study by Avshalom Caspi, Ph.D., and colleagues that appeared in Science in 2002 demonstrating a remarkable interaction between a specific genetic configuration and early childhood experiences in the development of antisocial disorder.

Drawing on a sample of more than 400 males in Dunedin, Scotland, who had been followed since childhood for 26 years, Caspi and colleagues were able to examine the levels of monoamine oxidase A (MAOA) activity in those who did and did not exhibit antisocial behavior, including violence, in later years.

MAOA is an enzyme that sits on mitochondrial membranes in neurons and degrades several important neurotransmitters, including several believed to be important in the regulation of aggression and impulsivity. Previous animal research had shown that the absence of MAOA was associated with increased aggression. Levels of MAOA activity differ based on variation in the "promoter region" of the MAOA gene, which controls the transcription of the DNA into messenger RNA.

Caspi and colleagues found from their longitudinal work with the Dunedin sample that low MAOA activity was not itself predictive, but that low MAOA activity in combination with a history of child abuse or neglect was predictive of antisocial behavior, including violence. Individuals with low MAOA activity and severe maltreatment comprised just 12 percent of the sample, but they accounted for 44 percent of the violent crimes committed by the sample. . . .