A group of 20 brain-damaged (BD) subjects was added to the canonical analysis, reported earlier, of mandibular symphysis shape, in the following groups: (1) Down's syndrome (DS), (2) familial idiopathic sub-normality (FIS), (3) intermediate intelligence (II), and (4) high intelligence (HI). Significant separations between means were found for all pairs of groups, except only II–HI. The separation between the BD and FIS groups supports the hypothesis that a genetic mechanism (pleiotropy) underlies the link between intelligence and symphysis shape, but environmental influences probably account for most of the separation between the BD and the two normal groups. The findings have implications for the study of behaviour genetics