ES-B: According to the one-factor account of delusion formation, we need only appeal to an anomalous experience to explain why a subject comes to hold a delusional belief, it is only the kinds of experiences subjects with delusions have which sets them apart from the non-delusional population. What do you think is wrong with this account?
MC: Whenever you identify an anomalous experience that you think is the cause of a delusion, you can always find patients who have that experience but are not deluded. We have done that systematically. We just go through a whole lot of different monothematic delusions—about eight or nine of them—proposing a first factor, showing that that’s a plausible source of the content of the belief and for each of those first factors showing that there are patients that have that first factor who are not delusional. And so I think that rules out a one-factor explanation for all of those monothematic delusions.
ES-B: Could the one-factor approach appeal to individual differences here? If the belief formation mechanisms of patients with delusions operate within what we might call the ‘normal range’, this might allow for a range of possibilities regarding forming beliefs upon certain experiences. What is wrong with that kind of line?
MC: Well, first of all you have to decide whether there are at least some cases where the second factor is due to brain damage, and I think you can’t deny that, and so it’s not very parsimonious, with that evidence, to appeal to it sometimes being individual differences. Then you’re saying that there are two different kinds of second factors. There’s one that’s down to brain damage, and there’s one that’s down to individual differences. The two problems with that is that it’s not parsimonious and there’s no evidence that any patients like this have a second factor that’s pre-morbidly present and down to individual differences. Now that doesn’t rule out this theory, you can never completely rule out a theory, just, why say it? The only reason to say it would be if you denied that the second factor was ever due to brain damage, and you can’t do that.