Criminality is an ancient behaviour that appears with the civilization of the human being. Indeed even though one can measure aggression and violence in animals it could never be associated to criminality, for this concept involves the notion of law and moral that one transgresses. This is why criminality just as a concept could never be a purely biological outcome, because its roots are founded in society and thus in environment. When studying such a subject one must remember that we are discussing a social problem as well as a psychological problem and a biological one.
Indeed we are going to discuss that different biological causes lead to criminality or predispose someone to it, then we will discuss the environmental factors. When discussing the genetics of criminality one must first specify what kind of criminality is in question. Indeed it would be hard to talk about the possible genetic inference in fraudulent behaviour. This is why we want to make a differentiation between violent criminality and non violent criminality.
We are going to discuss the possibility of genes having some influence in criminality because "though the belief that single genetics defects produce criminal behaviour is untenable, behaviour is regarded as an emergent function of physical systems and brain activity which provides the causal (generative) mechanisms for it and constrains its form" ( Koutouvidis, Minogianni, 2003). The first studies into aggressiveness, were studies on mice and their likely hood for attack.
The main finding in research with mice and aggressiveness was that serotonin had some kind of impact on aggressiveness. Sandon et al. in 1994 found that very aggressive mice were deficient in serotonin receptors, thus leading to a lack of serotonin intake by the neurons and lesser levels of serotonin in the brain which lead to aggressiveness. This hypothesis was supported by Vazelli (1973), who found that mice in social isolation were extremely aggressive towards other males when put back into social context (with other mice) and very likely to attack.
The study found that serotonin turn over decreased when being in social isolation thus finding the correlation between low serotonin turnover and aggressiveness in mice. When a comparing study was undertaken with female mice this correlation did not occur, indeed female mice did not suffer from social isolation, the serotonin turn over levels stayed equivalent whether in social context or isolation. Moreover when the female mice were put back into the group of mice, no aggressive behaviour was apparent.
One can definitely observe the correlation between serotonin and aggressiveness, more so when looking at a study by Higely et al. in 1996 that studied monkeys and found significance in such a correlation, indeed the most aggressive monkeys where those with the lowest serotonin turnover. However, one cannot infer causality between violent behaviour and serotonin turnover in the brain, indeed no significant results show that it could be a cause to violent behaviour.
There is no direct effect on criminal behaviour showed. However a study by Bernan and Hen in 1997 showed that serotonin was linked with the decrease of inhibitions and impulses. This would mean that individuals are not by nature more aggressive than others but that some find difficulty in inhibiting these impulses to be aggressive. It has been found that low serotonin levels is also linked to depression (Daidson, Putman, and Larson, 2000) so would this mean that depressive individuals will tend to criminality?
This correlation seems to be absurd; this is why the study showed that serotonin was linked to inhibition and maintaining mood, not directly to criminal behaviour. This would mean that low serotonin levels would have different effects on different people depending on their background. Individuals predisposed to depression, would be affected by low serotonin turn over in a different way to individuals predisposed to aggressive behaviour. When looking into the biological factors of criminality one must look in to heritability, it being the most effective way to observe genetic similarities between individuals.
These studies usually involve studying twins, monozygotic twins being more effective, for the genetic resemblance is extremely high, or studying adopted children and the difference in behaviour to the biological parents. When considering criminal behaviour monozygotic twins resemble dizygotic twins and adoptive children resemble biological parents more than adoptive parents (Mason and Frick, 1994). However Jay (2001) sated that adoption studies of criminal behaviour where "confounded by placement factors". This meaning that in the study of criminality, studying twins for heritability would be more effective than adoption studies.