In the next four chapters, we will examine biological, psychological, and social factors for the personality disorders. Any of these could be associated with mental disorders: if they make the illness more likely, they are risk factors; if they make the illness less likely, they are protective factors.

In a diathesis-stress model, diatheses are inborn individual differences that influence the vulnerability to mental disorders. The biological risk factors to be examined in this chapter could be diatheses for personality disorders. However, they do not, by themselves, explain their etiology.

The biological factors in mental illness can be measured either through genetic studies, or by the identification of biological markers.

Genetic predispositions to psychiatric disorders are identified by family history methods, by adoption studies, or by twin studies. Family history methods, which determine how frequently a disorder is found in the close relatives of patients, provide information that can only be suggestive of genetic influence, since they cannot separate heredity from environment. On the other hand, adoption and twin studies offer much stronger evidence for biological factors. Adoption studies examine whether the children of parents with psychiatric disorders will develop the same disorder if raised in another family. Twin studies, which determine whether identical twins are more concordant for a disorder than are fraternal twins, are the most common way to measure heritability.

Biological markers are an indirect measurement of heritability. Although they could, in principle, reflect either the causes or the consequences of psychopathology, their presence, particularly in combination with other evidence, points to genetic diatheses in mental disorders (Gottesman, 1991).