A few sentences should be mentioned concerning the etiology of personality disorders. There is not much empirical evidence concerning the causes of these disorders, and they are often quite hard to diagnose for several reasons. One reason is that personality disorders are polythetic; there is a lot of heterogeneity, which makes it difficult to account for false positives. Secondly, there is a great overlap among the personality disorders and also between personality disorders and the axis I disorders. Thirdly, the diagnostic criteria for many of these disorders require a subjective determination of whether or not a symptom is present, which, except for ASPD, has a very low rate of reliability. The final problem with classifying and understanding personality disorders is that the "persistent" element of the definition of a disorder is not always true. There is very low predictive validity because, in truth, we are only seeing the person at one point in his or her life. In general, low reliability and validity make determining the etiology of these disorders difficult (except, again, for ASPD, which has strong patterns of reliability and clear etiology).