Delusional disorder (DD) is an under-researched condition and its relationship to schizophrenia (SZ) controversial. This study aimed to further characterize DD and to examine multi-domain evidence for the distinction between DD and SZ.

Using univariate analyses we examined 146 subjects with DD, 114 subjects with paranoid SZ and 244 subjects with non-paranoid SZ on 52 characteristics from several domains including demographics, risk factors, premorbid features, illness characteristics, index episode features, delusional-related features, response to treatment and outcome. In a further step, we searched for independent associations of the examined characteristics with DD v. SZ.

Univariate analyses showed that DD differed from either form of SZ in 40 characteristics, the pattern of findings indicated that paranoid SZ was much more similar to non-paranoid SZ than DD. Relative to subjects with SZ, those with DD were more likely to have drug abuse before illness onset, better premorbid sexual adjustment, later age at illness onset, higher levels of affective symptoms and lack of insight, poorer response to antipsychotic medication, better functioning in the domains of personal care, paid work and social functioning; last, subjects with DD had fewer but more severe delusions and higher ratings of conviction of delusional experience than those with SZ. Predominance of jealousy and somatic delusions was confined to subjects with DD.

DD and SZ represent two distinct classes of disorders, the differential features of DD being of nosological, aetiological and therapeutic relevance.