Contributions to variance in phalangeal growth: Estimates from twins

Lengths of the second metacarpal and fourth middle phalanx of a sample of 99 sets of radiographs of twins (45 MZ sets and 54 like‐sex DZ sets) are subjected to variance analysis. The purpose is to examine incremental growth status between the appearance of these bones and the fusion of their epiphyses. Grouped data show higher F‐ratios and H scores for males in MC2 length and for females in MP4 length. When the data are sub‐divided by age (10 and under, older than 10), the later ages show uniformly higher F and H scores than the younger, and males higher than females for all measurements. It is suggested that the influence of the environment is relatively less in later ages, and that X‐chromosomal influence plays a part in determining tubular bone growth in the hand‐wrist.