• A group of 46 bipolar probands was divided equally into those with early (before age 30) and later onset. The only significant difference found was a higher morbidity risk for the relatives of the early-onset group. Slater's model for a polygenic transmission was found compatible with this group, but insufficient data prevented its application in the older-onset group. In all other respects, however, no significant differences were found.
Age of onset of illness may be unhelpful in elucidating the genetic basis of affective disorder. Further progress will depend on the development of more sensitive mathematical models and the finding of specific genetic markers.