Researchers scientifically investigate the effect of genetic influence through the use of family and twin studies. These studies, coupled with the recently developed technologies of neuroscience, have led to the fairly solid conclusion that many personality traits (i.e., dispositions to behave in particular ways across situations and time) are inherited and biologically determined (Cloninger, 2005).
Coccaro and Siever (2005) reviewed many neurobiological research studies and reported their summary conclusions about the neurobiological factors associated with personality disorder. Here we will touch upon some of their most intriguing conclusions:
Quite a large number of studies have looked at the genetic factors associated with Schizotypal Personality Disorder. From theses studies we have learned that Schizotypal Personality Disorder tends to be more frequent in families where at least one family member has been previously diagnosed with Schizophrenia (Coccaro & Siever, 2005). As well, these studies have documented similar, distinctive, behavioral patterns (such as peculiarities in startle or eye movement response) in family members of both disorders. This finding suggests these behaviors share a common genetic origin (Coccaro & Siever, 2005). Other family studies have revealed impulsive aggression tends to run in families of people diagnosed with Borderline Personality Disorder, again suggesting a genetic origin for this behavioral tendency (Coccaro & Siever, 2005).
While family studies may suggest the genetic transmission of particular behavioral tendencies, these research studies cannot be easily designed to rule out the possibility that such tendencies are transmitted through learning mechanisms. In other words, children may learn to be aggressive simply by observing other family members behaving that way. Thus, it is important to look to alternative research methods in order to tease apart genetic influence, from environmental influence.
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Fortunately, neurological studies of brain chemistry and brain anatomy in individuals with personality disorders provide us with corroborating evidence for the important role of genetics in the formation and maintenance of certain behavioral tendencies. Consistent with the findings of family studies, neurological studies have indeed identified brain abnormalities in people with Schizotypal Personality Disorder (Coccaro & Siever, 2005). These abnormalities are located in an area of the brain called the temporal cortex. These abnormalities are very similar to those observed in the brains of people diagnosed with Schizophrenia. It is suspected that the superior functioning of other brain regions protects people with Schizotypal Personality Disorder from developing the more debilitating psychotic features (hallucinations and delusions) associated with Schizophrenia.
Neurological studies have also provided corroborating evidence for the role of genetics with regard to impulsive aggression. Coccaro & Siever (2005) reported that researchers have identified a relationship between impulsive aggression and a complex, neuro-chemical system called the serotonergic system. This finding suggests there is biological, neurological etiology for the symptom of impulsive aggression that is characteristic of Cluster B personality disorders (Antisocial, Histrionic, Narcissistic, and Borderline Personality Disorders). Furthermore, this finding has led to some promising pharmacological interventions.
Neurological research studies have also documented that people with Cluster B personality disorders demonstrate problematic functioning of their prefrontal cortices (Coccaro & Siever, 2005). Damage to this vital brain region can limit people's ability to exercise good judgment and to think their way out of negative moods. Given this data, it seems an unlikely coincidence that people with Cluster B personality disorders often report difficulty tempering their powerful emotions with rational thought, thereby making it more difficult for them to make the wisest choices.
From such studies, it becomes fairly clear that people with personality disorders do not just act badly on a whim. Abnormal brain functioning and genetic loading make it more difficult for them to act in a different, more functional manner (relative to their more healthy peers). These studies do not definitively establish that brain impairments "cause" personality disorders, or even that they exist due to genetic "design" (versus environmental acquisition) but they do trend in that direction. Either way, these findings are compelling.
The influence of biology on behavior is also revealed in the phenomena of temperament, or inborn personality disposition. As any parent will attest, children are born already possessing primitive personality tendencies. Some children are born with milder, calmer dispositions while others are more anxious and irritable. These inborn personality tendencies (temperaments) are assumed to be largely influenced by genetic factors in a manner analogous to how hair and eye coloration are genetically determined. More specific information about biological influences is discussed in the section on the neuro-behavioral explanations of personality disorders.