As discussed in chapter 1, we have used a score of 27 on the APSD as our cut-off point for a classification of psychopathic tendencies in many of our studies.

I've noticed this elsewhere (looking into ADHD), Psychiatrists seem interested in developing a criteria which seems naturally continuous, and then using a cutoff without arguing for why that's a good idea. I can easily imagine that some conditions are discrete, but many of them must be pretty continuous. It seems like they would lose a lot of statistical power with a cutoff approach.

Is this purely a historical accident? Is it because discrete judgments seem more authoritative? Is there an actual good reason that I can't see? What's going on here? This sort of thing makes me suspicious of the quality of psychiatry research.