First of all, criminal behavior can have a variety of definitions depending on different perspectives, it also constitutes various activities. Therefore, psychologists normally take criminal behavior to a broader context as antisocial behavior (Jones, 2005). Morley & Hall, (2003) suggested three ways to define antisocial behavior. First it involves criminality and delinquency, which both engaging in unlawful acts. Criminality is the state of being a criminal as adults, whereas delinquency refers to violation of the law by a minor (WorldNet Dictionary, 2012). Secondly, antisocial behavior is defined from the general population through measurements used to diagnose certain personality disorders. For example, Antisocial Personality Disorder (also known as psychopathic personality), which is ‘‘characterized by amorality and lack of affect; capable of violent acts without guilt feelings’’ (WorldNet Dictionary, 2012). The third measurement of antisocial behavior is those influential personality traits which may induce such criminal behavior in individuals. The most typical traits are aggressiveness and impulsivity (Morley & Hall, 2003, cited in Jones, 2005).
Many researchers believe that by understanding the biology will be essential in predicting individuals which are predisposed to perform criminal behaviors. Biological explanations attribute criminal behavior to internal factors and put its emphasizing on the nature side (Jones, 2005). Borgaonkar and Shah, (1974 cited in Alper, 1995) first assumed that an extra Y chromosome in males was the cause to violent criminal behavior; however later studies could not replicate this finding. Recently, genetic, biochemistry and brain imaging studies have gained widespread popularity and have all found convincing evidence as the biological explanation for criminal behavior. This essay will describe and critically evaluate these findings especially from genetic and brain imaging studies.
After Mendel (1866, cited in Feldkamp et al., 2002) proposed the concept of genetically inherited traits. Twin and adoption studies have been the major methods in researching genetic influences on criminal behavior. Twin studies are studies that compare the rates of criminal behavior between identical (monozygotic, meaning derived from a single fertilized egg) twins and fraternal (dizygotic, meaning derived from two separately fertilized eggs) twins. The genetic influence is proved if there is a higher concordance rate for MZ twins than for DZ twins in criminal behavior (Tehrani & Mednick, 2000, cited in Jones, 2005). A longitudinal study of 85 MZ twin pairs and 147 DZ twin pairs found that there was not only a higher concordance rate for the MZ twins but also a higher tendency toward committing criminal acts ten years later (Joseph, 2001, cited in Jones, 2005). The results from another study contradicted the finding; researchers looked at 49 MZ twin pairs and 89 DZ twin pairs but did not find any significant difference in the concordance rate. They argued that there is no significance of genetic influence to violent crimes; besides, they also suggested that twin studies fail to separate genetic and environmental influences (Joseph, 2001).
Adoption studies assume that children may inherit the genes from their parents with a predisposition to antisocial behavior and these children are more vulnerable than the others in developing similar antisocial behavior. Therefore, adoption studies are assumed to be better in separating genetic and environmental influences. They are useful in examining the relationship between adopted children and both their biological and adoptive parents with regardless of social and environmental influences (Jones, 2005). The adoption study conducted by Tehrani & Mednick, (2000) found that those adopted individuals with a biological criminal mother had a higher rate of criminal offending in adulthood. Despite the consistent evidence showing a relationship between adopted children's antisocial behavior and their biological parents' criminal behavior, this relationship can only be found in the case of property crime, but not in the case of violent crime (Brennan, Mednick & Jacobsen 1996). As stated by Joseph, (2001, p1), ''no adoption researcher claimed to have found evidence of a genetic predisposition for violent crime.'' Despite some methodological problems and controversial findings, scientists agree that genes play an important role in antisocial behavior; overall, evidence from twins and adoption studies conclude that ''there are about 50% of the variance in antisocial behavior is attributable to genetic influences'' (Moffitt, 2005, p554).
On the other hand, molecular genetic studies found that there are specific types of genes which are associated with antisocial behavior. For example, monoamine oxidase A (MAOA) gene; it was firstly found to be involved in aggressive behavior in mice (Cases et al., 1995, cited in Raine, 2008). The MAOA genes enzyme breaks down a kind of neurotransmitter called serotonin, and antisocial individuals generally have low levels of serotonin (Raine, 2008). Later, there was evidence coming from human studies. The research conducted by Capsi et al., (2002) studied maltreated children in their development of antisocial behavior, and they found that those children with high levels of MAOA expression in their genotype had fewer tendencies to develop antisocial behavior. A replication study on 975 boys done by Kim-Cohen et al., (2006) also found strong evidence supporting the association between the MAOA gene and antisocial behavior. Furthermore, Meyer-Lindenberg et al., (2006) found that the polymorphism (meaning the existence of different kinds of crystal of the same chemical compound, from WordNet Dictionary, 2012) of the MAOA gene in males can result in the volume reduction of the amygdala, anterior cingulate and ventral prefrontal cortex. These brain areas are involved in emotion. The challenge on the research of certain type of genes is that until now there are already 7 genes at least (MAOA, 5HTT, BDNF, NOTCH4, NCAM, tlx, and Pet-1-ETS) have found to be in association with antisocial or aggressive behavior and of are considered to influence brain structure; MAOA gene is just one of them (Raine, 2008). Therefore, further investigations are necessary in the field of biochemistry for criminal behavior.
With the increasing use of brain imaging techniques, brain imaging studies have revealed both structural and functional impairments in antisocial individuals. The most commonly used functional imaging techniques include positron emission tomography (PET), functional magnetic resonance imaging (fMRI) and anatomical magnetic resonance imaging (aMRI) for structural imaging studies (Yang et al., 2008). Some of the PET studies measure antisocial individuals' glucose metabolic activities (the level of radioactivity) in resting status and then compare with control subjects. For example, Volkow et al. (1995, cited in Yang, Glenn and Raine, 2008) found that Antisocial Personality Disorder patients had abnormal glucose metabolism in prefrontal and medial temporal regions. Later Wong et al. (1997, cited in Yang et al., 2008) found lower glucose metabolism in the anterior inferior temporal cortex in violent offenders with schizophrenia. Other PET studies measured the glucose metabolism abnormality in antisocial individuals while doing a continuous performance task (CPT). Researchers found reduced metabolic in the anterior medial prefrontal, orbitofrontal, and superior frontal cortex in murderers (Raine, Buchsbaum, Stanley, Lottenberg, Abel & Stoddard, 1994, cited in Yanf et al., 2008); George, Rawlings, Williams, Phillips, Fong, Kerich, Momenan, Umhau & Hommer, 2004, cited in Yang et al., 2008) observed significantly lower glucose metabolism in the right hypothalamus in violence perpetrators with alcohol dependence. This technology has the limitation of injecting the participants a short-lived radioactive tracer prior to the scan; therefore, ''making it somewhat less suitable for use in community samples'' (Yang et al., 2008, p72).
A better development in functional imaging technology is the use of fMRI. This technique does not require any injection. It measures the abnormal neural activity in the brain of antisocial individuals while performing emotional or cognitive tasks. Researchers found reduced activity in the amygdala-hippocampal complex (Kiehl, Smith, Mendrek, Forster, Hare & Liddle, 2004, cited in Yang et al., 2008) and also in the right dorsal anterior cigulate cortex (Sterzer, Stadler, Krebs, Kleinschmidt & Poustka, 2005, cited in Yang et al., 2008). However, researchers also observed increased activation in the frontotemporal and the right prefrontal regions (Yang et al., 2008). Overall, a large number of studies used the relatively new fMRI technology, together they provide important evidences indicating functional dysfunctions in the frontal, temporal, and limbic regions in antisocial individuals.
The structural imaging technique aMRI can be used to detect volumetric abnormalities (i.e. increased or reduced gray matter volume) in antisocial individuals when compare to the controls (Yang et al., 2008). Many researches have observed the reduced gray matter in the prefrontal cortex in antisocial behaviour individuals (Raine et al., 2000; Woermann et al., 2000; cited in Yang et al., 2008). More specifically, Laakso et al., (2002, cited in Yang et al., 2008) found reduced gray matter volume in dorsolateral prefrontal, medial frontal, and orbitofrontal cortex. Other studies found a volume reduction in the temporal as well as prefrontal cortex (Barbas, 2006; Squire et al., 2004, cited in Brito et al., 2009). These brain regions are thought to be involved in sensory, affective, and higher cognitive processing. There are also studies of antisocial individuals shown structural deficits in the subcortical regions, particularly the hippocampus and corpus callosum (Laakso et al., 2001; Raine et al., 2003; cited in Yang et al., 2008). However, this conclusion is still plausible and requires future studies.
In conclusion, evidence from twin and adoption studies shows the attribution of genetic influence in antisocial behaviour, but this variance is estimated to be only 50%. Therefore, genetic studies does not support the idea that a genetic basis for any type of criminal or antisocial behavior (Joseph, 2001). Molecular genetic studies found the relationship of certain types of genes with antisocial behaviour, e.g. MAOA gene, but there are already 7 genes found to date, which all need further investigation. A large amount of evidence comes from brain imaging studies, and these studies have shown structural and functional abnormalities in antisocial individuals suggesting the link between criminal behavior to deficits in the prefrontal cortex, temporal cortex, insula, amygdala, hippocampus (para-hippocampus), and anterior/posterior cingulate gyrus (Yang and Raine, 2009). However, it should be noted that very few if any studies examine the sample for both structural and functional imaging. Also studies should be aware of the heterogeneity of the antisocial individuals; in addition, ''violence, psychopathy, and comorbid psychiatric disorders may moderate study outcomes'' (Yang & Raine, 2009). Furthermore, future brain imaging research could focus on other less studies regions, e.g. amygdala, hippocampus, insula, angular gyrus (Yang & Raine, 2009).