The purposes of this paper are to study the appearing frequencies of the bilateral mesiopalatal rotation of upper central incisors among the Japanese and the Pashtun, and to determine the genesis of this feature. The author observed the mesiopalatal rotation of upper central incisors by DAHLBERG's classification and measured the rotative angle by ROTHHAMMER's method. Relationships between the rotative angle and the size of front teeth, dental arch, palate, upper protrusion, overbite, molar relation or space available for two incisors were also investigated. The materials of the Japanese and the Pashtun were the plaster casts. The results are summarized as follows:
1. The rotative angle is larger among the Japanese than among the Pashtun. And the frequency of the mesiopalatal rotation is higher among the Japanese than among the Pashtun.
2. The positive relationships between the rotative angle, and the breadth of dental arch and of palate were observed. The negative relationship between the rotative angle and the length of anterior dental arch was observed. No significant relationships were found between this feature and the above factors, such as size of front teeth, space available for incisors, upper protrusion, crowding and spacing,
3. The evidences adduced above suggest that this feature is not the same phenotype of tooth crowding, and the genesis of this feature is not due to local factors but due to heritable factors.
The Journal of the Anthropological Society of Tokyo