Crime Causation: Biological Theories

Serotonin (5-HT; 5-hydroxytryptamine), a neurotransmitter produced by the raphe nuclei, is thought to be involved in the modulation of impulsivity. Consequently, serotonergic dysregulation may result in a decreased ability to inhibit certain externalizing behavioral patterns and may reflect a deficit in behavioral inhibition. It seems reasonable to hypothesize that violent criminal behavior, an outcome often marked by behavioral disinhibition, may be linked to some type of dysregulation of the serotonin system. A review of biochemical studies that have investigated the role of low serotonin concentrations in the emergence of criminal behavior follows. These studies have primarily examined levels of the cerebrospinal fluid (CSF) 5-HT metabolite, 5-hydroxyindolacetic acid, CSF 5-HIAA.

Recently, an impressive body of evidence, primarily obtained from biochemical studies, has accumulated regarding the role of the serotonin system in criminal behavior. Linnoila and colleagues have reported that within the context of a Finnish forensic population, violent offenders and impulsive fire-setters evidenced lower mean CSF 5-HIAA than normal controls (Virkkunen et al., 1989). This seems to suggest that serotonin dysfunction may play an etiologic role in more severe forms of antisocial behavior, such as violent offending. These studies have been extended to investigate whether serotonin levels can differentiate offender populations based upon type of the index offense and the presence or absence of alcohol abuse and violence in first-degree family members.

Virkkunen and others (1996) report that a combination of paternal violence and alcoholism, as measured by questionnaires to the first-degree relatives, was associated with low CSF 5-HIAA concentration levels in the male subjects, irrespective of subgroup classification (i.e., impulsive vs. nonimpulsive). The authors suggest that a familial trait may be associated with early-onset alcohol abuse, violent and impulsive offending, and low CSF 5-HIAA concentrations.

Subjects who had committed violent crimes during the 4.5-year follow-up period had lower CSF levels compared to nonrecidivists. Moreover, violent recidivists were more likely to have experienced paternal absence than nonrecidivists, suggesting the importance of both biological and environmental factors in the prediction of recidivistic violent offending. Due to the highly selective nature of the sample, results must be interpreted cautiously. A significant relationship between aggressiveness, parental absence, and low levels of serotonin was also noted in a study of nonhuman primates (Higley et al., 1993).

Virkkunen and others (1994) reported that impulsive violent offenses and impulsive fire-setters were found to evidence lower CSF 5-HIAA concentration levels; violent alcohol offenders whose index crime was not found to be impulsive had normal CSF 5-HIAA concentrations. The emphasis on the index offense as opposed to the qualitative nature of the cumulative criminal history, however, may be interpreted as a weakness of this study. On the basis of these findings, Virkkunen and colleagues propose that low serotonin may be a biological marker specific to impulsive violent offending accompanied by alcoholism. These conclusions, however, are drawn from a subject pool of forensic patients, representing a sample of heavily violent individuals. Within the context of a community sample, Hibbeln and others found that relative to the nonviolent control group, the violent group evidenced significantly lower concentration levels of CSF 5-HIAA.

One of the limitations of the biochemical studies is that CSF metabolites reflect presynaptic neurotransmitter activity; therefore, it is not known what is occurring at the postsynaptic level. Apart from the lack of specificity in information, efforts to investigate the role of serotonin in behavioral outcomes in humans have been challenging due to the fact that CSF levels of serotonin are collected via a lumbar puncture. More importantly, examination of the CSF does not provide information about the role of specific brain regions. Results from neuropsychological measures, for example, have consistently found neurological deficits to be present among antisocial persons than in nonantisocial persons. The limitation of neuropsychological indices, however, is that they present an indirect measure of brain functioning. Other, more recent techniques, have been applied to uncover the structural and functional properties of the brain in relation to criminal behavior. Brain imagining techniques, for example, have received an increasingly prominent role in the study of criminal behavior. These recent advances may in fact represent an important sector of the future of biological research in the field of criminal behavior.