In the meantime, new brain cells are constantly being developed, even into old age, but such alterations in the brain are largely contingent on what the individual has experienced in his or her environment" (Wright et al., 2008). Therefore, one of the factors that can impact adult propensity to commit violence is whether the brain had the opportunity to develop normally in very early childhood. Of course, both social and biological factors can impact early brain development. In many ways, criminals are not thought to respond to external stimuli in the same manner as non-criminals, and it is believed that there may be an underlying biological basis for these differences. The auto-nomic nervous system (ANS) is the part of the nervous system beyond the conscious control of the individual and is split into the sympathetic nervous system and the parasympathetic nervous system. The sympathetic nervous system prepares the body for action while the parasympathetic nervous system handles baseline tasks. In many criminals, the relationship between the two parts of the ANS operates differently than in non-criminals, with them exhibiting lower hear rates. These lower heart rates are thought to be the result of "a phenomenon known as vagotonia, which is a predominance of the parasympathetic autonomic processes over the sympathetic system. Vagotonia is thought to occur from overstimulation of the vagus nerve, which encourages the release of insulin from...

This is also thought to lead to slower brain-wave functioning. Therefore, these biological factors are linked to criminality.
From a neurological standpoint, reward dominance theory helps explain criminal behavior by looking at criminality as a form of addiction. "Our motivation and our sensation-seeking behavior are due to the interplay of the left frontal lobe, mesolimbic structures, and chemicals, most notably dopamine. The BAS creates the drive in us to seek out essentials of life, such as good food or a love interest…Unfortunately, this system, if unchecked, creates an excessive and unhealthy drive to seek pleasurable sensation (some have called this the "craving brain") (Ruden, 1997), even if it means engaging in criminal activity, such as drug use or chronic gambling. (Wright et al., 2008). There is a relationship between addiction and serotonin levels, and there are links between low levels of cerebrospinal serotonin both violence and suicide. Genes that are linked to either serotonin production or the body's ability to utilize serotonin can increase criminal propensity, as low levels of serotonin are linked to increased risk of criminal behavior.

References

Wright, J.P., Tibbetts, S.G., & Daigle, L.E. (2008). Criminals in the Making . Thousand Oaks:

Sage Publications Inc. .