Current research suggests that the genetic aspects of aggression and criminal behavior are multifactorial.  Of the many human studies that have been carried out, including adoption and twin studies, on the influence of genetics on antisocial and aggressive behavior, no researcher has been able to pinpoint a genetic source that could be applied to a general population.  Animal studies, carried out mainly in mice, have proven effective in identifying candidate genes or systems from which some conclusions can be drawn.  However, the broad applicability of animal studies is questioned in relation to human behavior because of the nature of the controled animal experiments.
 
 

XYY Syndrome

In the 1970s, great attention was given to studies that concluded that there was a high prevelance of XYY syndrome among various types of institutionalized people.  These studies suggested that this chromosomal abnormality was associated with aggressive, antisocial, and criminal behavior.  However, prospective studies  and population-based studies soon dismissed this syndrome as being the sole cause of such behavior with more emphasis on developmental processes being responsible.

Monoamine Oxidase-A (MAO-A)

MAO-A is a mitochondrial enzyme responsible for the breakdown of several neurotransmitters including serotonin, dopamine, and noradreniline.  A study by Brunner et al. in 1993 identified a Dutch family with an inherited deficiency resulting from a stop codon in the gene sequence.  Males with this deficiency from a variety of environments showed altered behavior including shyness, withdrawn, and unprovoked or minimally provoked aggressive outbursts.  While this gene deficiency was present in high frequencies in this one kindred, the population frequencies of deficiencies of this nature are unknown but are predicted to be extremely low.  Therefore, the usefulness of this study lies not in its ability to predict aggressive behavior in a population but in the direct demonstration that genes can influence behavior, a notion that seems logical.

Other Candidate Systems and Genes

There are currently over a dozen genes that are being investigated which have a potential to yield insights concerning the nature of aggression.  Dopamine has been implicated in impulsive and aggressive behavior, particularly in schizophrenia and alcoholism.  Additionally, low levels of serotonin (5-hydroxytryptamine, 5-HT) have been found in victims of violent suicides, impulsive aggressive individuals, and in certain kinds of alcoholism.  Both serotonin and dopamine are neurotransmitters that have many identified and as yet unidentified roles at both physiological and behavioral levels.  Many of the genes being investigated for a role code for receptors or enzymes in the metabolic pathways of these molecules.  Such genes include MAO-A, MAO-B, tryptophan hydroxylase (TPH), an enzyme that catalyzes the rate-limiting step in serotonin biosynthesis, and at least four serotonin and three dopamine receptors.  The current knowledge of the genetic determinants of aggressive behavior has at best provided more questions than answers.  What are the predictions for the future in this line of research?

First and foremost, the idea of an "aggression gene" does not make sense.  Altered behavior is something entirely different than an altered physiological response or disease.  While one gene and its protein product could be ultimately responsible for a diseased state, this is almost entirely improbable for such effects on behavior.  Behaviors arise from the highest levels of brain organization and for even a single behavior to be controlled by a deterministic single gene is illogical.  It is generally accepted that a deterministic view of behavior is wrong and that a multitude of genetic and non-genetic variables control the propensities of individuals to exhibit aggressive or impulsive behaviors.  There are many compelling reasons to doubt the existence of specific genetic markers to predict aggression.  First, research thus far indicates that such a discovery is unlikely because of the many possible environmental influences on an individual.  Second, genetic effects for aggressive behaviors are highly malleable over the course of development and the timing of factors (e.g. drugs, hormones) effecting development has a significant impact on abnormal development.  Finally, the genetic influences on aggressive behavior thus far appear to be dynamic, changing with age and between individuals.

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