Patrick, C.L., & DiLalla, L.F. (1995). Development of handedness patterns in infant twins suggests heritability. Paper presented to the Behavioral Genetics Association, June, Richmond, VA. Abstracted in Behavior Genetics, 21, 282.

Examining the heritability of handedness in infants and young children can be difficult, because there is considerable instability in both direction and strength of handedness. Only 10% of children show a clear preference by age 1, and 30% by age 2 (Rice, Plomin, and DeFries, 1984, Perceptual and Motor Skills, 58, 683-689). Perhaps the more salient question when examining hand preference prior to age 2 is whether the pattern of change and development in handedness shows heritability. The present study examined handedness in 28 MZ and 37 DZ twin pairs at age 7 months and again at 9 months. Subjects were part of the Twin Infant Project (TIP; L.F. DiLalla et al., 1990, Developmental Psychology, 26, 759-769). Handedness was measured by the total amount of time in milliseconds subjects manipulated an object in each hand over five trials, when the object was offered at midline. The difference between the 9-month and the 7-month scores constituted the ""change" score for each infant. There was virtually no difference between the handedness scores at either 7 or 9 months. However, an heritable influence on the pattern of change from 7 to 9 months was evident. The MZ correlations were .32 for the right hand and .32 for the left hand, whereas DZ correlations were -.19 and .02, for the right and left hand, respectively. These results suggest that the actual pattern of change in handedness during infancy is heritable. It may be this developmental change, rather than static and nonstable handedness measure per se, that should be the focus of future research on infant handedness.