Schug et al's article on psychophysiological and behavioural characteristics of individuals with comorbid antisocial personality disorder (ASPD) and schizophrenia-spectrum personality disorder (SSPD) opens the possibility of the existence of a new spectrum of personality disorders. But reading the article with care raises some questions.

First, the community-based study population which was recruited from temporary employment agencies appears to have a prevalence of personality disorder of the order of what we would observe in psychiatric in-patient units. ^{Reference De Girolamo, Dotto, Gelder, Lopez-Ibor and Andreasen2} The total prevalence in this study population was 52.4%, which is way beyond the prevalence rate in community-based samples. ^{Reference Samuels, Eaton, Bienvenu, Brown, Costa and Nestadt3} Conducting studies in such a population could lead to selection bias. By nature, individuals from such samples are more likely to have more severe and complex forms of the disorder. ^{Reference Ajetunmobi4} This might explain the high prevalence rate of combined ASPD/SSPD observed in the study. It would have been more interesting to know the number of individuals from this sample who have comorbidity between SSPD and other personality disorders such as narcissistic, histrionic and borderline.

Second, in the psychophysiological and behavioural characteristics of the four study groups I noticed a pattern: the comorbid ASPD/SSPD groups appear at the severe end of the spectrum, where abnormal characteristics are more frequently observed (four out of six characteristics studied in this research, namely self-reported crime and skin conductance frequency, amplitude and arousal) than the control group, which lies at the other end of the spectrum. Individuals in the ASPD and SSPD groups lie on either side of the middle of the spectrum.

A basic structural abnormality in the frontal cortex might cause these individuals to have higher psychiatric comorbidity (not only of ASPD and SSPD) and the abnormal characteristics identified might not be entirely explained by the co-occurrence of these two disorders. The authors have rightly identified that the results could not merely be due to the additive effects of both disorders, but that the increased prevalence of personality disorder in the study population might be due to a common confounder that does not lie on the causal pathway between ASPD/SSPD and abnormality in characteristics.