Premorbid intelligence and educational level in bipolar and unipolar disorders: a Danish draft board study

Registry-based studies have found no or weak associations between premorbid intelligence and the broad entity of affective spectrum disorder, but none of the studies compared bipolar/unipolar subgroups.

IQ and educational level were assessed at the draft board, and hospital diagnoses were followed up to the ages 43-54 years for 294 individuals hospitalized with bipolar disorder and 1434 with unipolar or depressive disorder. Controls comprised 20,531 individuals without psychiatric registration.

Mean IQs of 98.32 and 96.71 were observed for patients with bipolar and depressive disorder respectively. For both patient groups a unimodal, slightly negatively skewed distribution was observed. The difference between the two patient samples was not statistically significant (p=0.10), but both obtained lower mean scores than the controls (p<0.0001 for unipolar and p=0.057 for bipolar patients). The means of the index of educational level were 5.22 and 4.82 for bipolar and unipolar patients (p=0.0006). The mean educational level was similar to the controls in bipolar disorder (p=0.15), while it was significantly lower compared to the controls in the unipolar group (p=0.0005). When possible prodromal patients were excluded, essentially the same results were observed.

The study was based on clinical hospital admission diagnoses.

Clinicians should be aware of the relatively high intelligence and educational level in patients with bipolar disorder compared with patients with unipolar disorder. However, we were unable to confirm distinct subgroups of bipolar disorder with high and low premorbid intelligence and educational level.