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The general definition of personality disorders (PD) has found agreement in many classification systems and has remained relatively stable over many decades. However, a closer look at the history of the classification of various PD reveals that there are changes from generation to generation: a continuous variation in our cultural norms means that some types are 'renormalised' from time to time and that others are added. On the other hand, a comparison of the descriptions and classifications of PD by Schneider, the DSM and ICD systems shows substantial agreement on many types of PD.

The International Diagnostic Checklists for the assessment of the DSM-III-R and ICD-10 personality disorders (IDCL-P) were compared with a structured interview, the International Personality Disorder Examination (IPDE), using a balanced test-retest design with forty psychiatric inpatients. The results, using pairwise kappa for the calculation of agreement, were as follows: any personality disorder versus no personality disorder 0.52 for DSM-III-R diagnoses and 0.75 for ICD-10 diagnoses. The range for the single personality disorders diagnosed at least five times was from -0.07 to 0.71 for DSM-III-R diagnoses and from 0.38 to 0.68 for ICD-10 diagnoses. Only for DSM-III-R diagnoses do figures exist from three other studies comparing two structured interviews with each other. The results of all four studies suggest that 60% of the variance in personality disorder diagnoses represents variance not attributable to the patients, which is scientifically unacceptable.

Two instruments for the assessment of the DSM-III-R personality disorders were compared: The Personality Disorders Questionnaire--Revised (PDQ-R) and the Munich Diagnostic Checklist for the assessment of DSM-III-R Personality Disorders (MDCL-P). Using kappa value as a measure of agreement, the diagnostic agreement was less than 0.40 for personality disorder vs. no personality disorder as well as for the specific personality disorders. The PDQ-R diagnosed more frequently personality disorders (58%) than did the MDCL-P (43%).