Abstract Over 80 per cent of the subjects showed some form of the character in the deciduous dentition, about 10 per cent having cusps; in the permanent dentition, about 70 per cent of the subjects had the character, 20 per cent showing cusps. Sexual dimorphism was not evident in the deciduous dentition but, in the permanent dentition, the character was more common in males who also showed a higher degree of expression than females. Generally, the trait occurred bilaterally with symmetrical expression in both dentitions. Although the character was present in the deciduous dentition but not the permanent in a number of subjects, the reverse situation was rare. Comparisons between siblings of individuals with the character and siblings of individuals from the general population provided some evidence of a genetic basis to the character, but its heritability is probably low. Variations in phenotypic trait expression between sides, sexes and dentitions probably reflect the interplay between environmental influences and the timing of developmental processes.