Published online by Cambridge University Press: 31 July 2008
The shape of the mandibular symphysis was studied in four groups of adult male Caucasians, aged 18–49. The composition of the groups and their numbers were as follows: (1) Down's syndrome (20), (2) familial idiopathic subnormality (20), (3) healthy individuals, IQ range 94−120 (22), and (4) healthy individuals, IQ 130+ (22). From lateral skull radiographs, the outline of the mandibular symphysis was digitized with a D-mac pencil follower. Selected distances and angles were computed and used as variables in a canonical analysis. Plots of the canonical variates discriminated appreciably between each group. Generalized distance measurements between group means were significant for all pairs of groups except 2/3 and 3/4. The results suggest a genetic link between intelligence and symphysis shape. This hypothesis will be tested by introducing a brain-damaged group, matched in intelligence, and for length of stay in hospital, with Group 2. The finding could influence developments in human genetics and psychology.