Birth weight and handedness in boys and girls.

The association between selected demographic variables and birth weight on the one hand and a composite hand preference score based on seven hand tasks (each performed twice) on the other was investigated in a sample of 1387 male and female schoolchildren aged 5 to 10 years old. In multiple regression models left-handedness was significantly more common among boys and among children of better educated mothers and tended to decrease with age. No association was found with respect to urban or rural residence or birth order. Increased birth weight was associated with right-handedness in boys but with left-handedness in girls, and the birth weight by sex interaction term was statistically significant (p = 0.037). The demographic associations in the present study are compatible with those reported previously. The different associations of birth weight with hand preference in boys and girls indicate that the prenatal hormonal factors that affect brain lateralization and handedness are qualitatively or quantitatively different in the two sexes and may be differentially associated with birth weight.