Paranoid personality disorder is marked by suspicion of other people's motives and intents. Individuals with paranoid personality disorder expect that other people are trying to harm them and take excessive precautions to avoid exploitation or injury. The prevalence rate for this type of disorder is between .5 and 2.5 percent. It is most frequently comorbid with borderline and avoidant personality disorders. Evidence indicates that paranoid personality disorders are most common among relatives of individuals diagnosed with schizophrenia; therefore, these individuals may be inheriting a strong genetic liability for developing some type of mental disorder. Another conclusion could be that the parents of the individual may somehow verbally or non-verbally communicate that other people should not be trusted. Some social factors associated with increased risk for this disorder are hearing impairments, and if an individual is a refugee, both factors which are thought likely to engender mistrust toward others.

Schizoid personality disorder is characterized by a pervasive indifference to other people, coupled with a diminished range of emotional expression. These individuals are detached from social relationships in that they prefer social isolation to spending time with friends or family. The prevalence rate for this disorder is about .7 percent. It is most comorbid with avoidant personality disorder and there is not much information concerning its course, outcome, or etiology.

Schizotypal personality disorder (SPD), considered by many as part of the schizophrenic spectrum, is characterized by discomfort with other people, peculiar patterns of thinking and behavior, and eccentric behavior. These may take the form of cognitive or perceptual disturbances. Yet, unlike schizophrenia, these psychotic symptoms are not as fully developed as delusions or hallucinations but instead can be characterized as perceptual allusions. The prevalence rate of SPD is about 3 percent. This disorder follows a chronic course, except for those individuals who go on to develop schizophrenia. It is mostly comorbid with paranoid and avoidant personality disorders.

Family, twin, and adoption studies all show an increased risk for developing schizotypal personality disorders amongst those individuals with a family history of schizophrenia. Some psychologists believe therefore, that there is a strong genetic diathesis for developing schizophrenia, yet in the absence of full-blown stressors, or triggers, the disorder takes the alternate form of schizotypal personality disorder. Proof for this comes from studies wherein pregnant women exposed to influenza epidemics gave birth to children with a higher risk of developing the disorder, indicatiing similar biological causes for schizophrenia and SPD. Furthermore, one Danish-American study found that children of schizophrenics not raised in schizophrenic households not only seemed to exhibit increased vulnerability towards developing schizophrenia, but also tended to inherit a cluster of symptoms that can only be defined as "strangeness". Ingraham defined these "strange," heritable features as suspicion, flat affect, and social withdrawal. Again, these studies serve as proof that schizophrenia and SPD may simply be different phenotypic expressions of the same genotype, and environmental factors then determine which one will manifest itself.

Further proof of this theory can be seen in such bio-behavioral markers as eye movement and skin conductance orienting response (SCOR). Empirical studies have found that individuals with SPD also have problems with eye tracking movements, just to a lesser degree than people with schizophrenia. Interestingly, certain things such as cognitive perceptual aberrations are also associated with eye tracking disorders. SCOR studies indicate that during the study, while "normal" people exhibit increases in electrical activity, a type of physiological change that occurs in the skin when a stimulus is changed, individuals with schizophrenia and SPD do not exhibit this selective criterion. This has led many researchers to believe that individuals with SPD may have problems with selective attention and that they may not be tuned to emotionally relevant stimuli.

Researchers have also found a positive correlation between HVA (a metabolite, or waste product of dopamine) levels and the psychotic symptoms associated with SPD. A final biological cause of SPD may come from looking at the HPA, or the hypothylamic-pituitary axis, which serves as a hormone relay station and thus plays an important role in maintaining stress levels. In individuals with SPD, HPA activity has been found to correlate positively with levels of anhedonia and social withdrawal.

Psychological and cognitive explanations of SPD focus on attentional and informational processing deficits. Researchers in this area have found that individuals with SPD perform poorly on continuous performance tasks, which assess one's ability to maintain attention on one object and measures the ability to selectively look at new stimuli, and therefore requires both vigilance and selective attention. SPD individuals also tend to perform very poorly on tasks consisting of emotionally-valenced words, indicating that they may possess a cognitive bias towards neutral words.

Two psychoanalytic theories have been offered to explain SPD. The first one concerns the concept of ego boundaries. ("Ego- psychologists" place a stronger component of the decision-making process on the ego.) For SPD individuals, then, there is conflict, or dysfunction between the outside and the inside world for the ego, thus leading to ego boundary problems. The second psychoanalytic theory stresses interaction with others, stating that SPD individuals existed in a state of high parental communication deviance. Evidence to support this theory comes from the TAT (thematic apperception test), which showed that parents of SPD patients tended to have strange communication problems and loose associations with words, regardless of whether or not they themselves had been diagnosed with schizophrenia or SPD. The higher their parents were in communication deviance, the worst the individual's symptoms and the more chronic the course of their disorder.

Medication such as traditional atypical neurolepticsand SSRIs have been effective in helping individuals with this disorder, but not to the same extent that they have helped individuals with schizophrenia (another indication of etiological differences between the two disorders). Psychological interventions usually involve attempting to change family dynamics and lower the rates of "expressed emotion" in the family, since studies have shown that high expressed emotion is positively correlated with rates of relapse. Psychoanalytic intervention focuses on defining ego boundaries. Cognitive behavioral therapy also yields effective results in that it not only attempts to help the individual to interpret and make sense of odd beliefs, but also teaches them valuable coping and interpersonal skills.