Belief inflexibility (BI) has been considered as a crucial factor for delusional conviction, but less is known about other dimensions of delusional beliefs. Question has been raised regarding the extent to which BI distinguishes delusions from strongly held (non-deluded) personally meaningful beliefs.

We examined the association between BI and major dimensions of delusional beliefs/non-clinical personally meaningful beliefs, and compared results from two BI measures (Maudsley assessment of delusions schedule [MADS] and bias against disconfirmatory evidence [BADE] task).

Idiosyncratic delusional beliefs from 40 outpatients with non-affective psychosis and personally meaningful beliefs from 30 healthy controls were assessed in an interview. Belief dimensions (conviction, preoccupation, and distress) and BI were measured.

Compared with controls, patients reported higher levels of distress and preoccupation but a comparable level of conviction (3.30/4 vs. 3.00/4, t(66.967) = 1.928. P = n.s.). Patients exhibited lower belief flexibility than controls on MADS but not on BADE. In patients, delusional conviction was associated with lower flexibility on a MADS item (“possibility of being mistaken”: t(38) = 4.808, P < 001) and the BADE evidence integration index (r = 0.463, P = 01). In healthy controls, belief conviction was associated with lower flexibility on a MADS item (“reaction to hypothetical contradiction”: t(27) = 3.345, P=.002). Two-way ANOVA revealed that the association between possibility of being mistaken and conviction was stronger in patients than controls (F(1) = 6.718, P = 012). In both groups, BI on either measure did not correlate with distress or preoccupation.

BI was specifically associated with belief conviction. The association was significant for both groups, and was stronger in patients than controls.

The authors have not supplied their declaration of competing interest.