Birth order, maternal age, season of birth, and handedness.

The handedness of 1,186 children about to enter kindergarten was tested. The sample represented approximately 98 percent of all children of this age in a school system during 8 years. Handedness was examined in relation to birth order, maternal age at birth, and sex, for 1,097 subjects (92.5%), and in relation to season of birth for the total sample. The finding that there were significantly more nonright-handed boys for birth orders one and four-or-later, than for two and three, provides support for Bakan (1971). There was a nonsignificant increase in nonright-handedness for boys born to the youngest (less than 20) and oldest (less than 30) mothers. A significant seasonal effect for the birth of nonright-handed boys was observed. For each of the fall and winter months (September - February) the proportion of nonright-handed male births was higher than that for any of the spring and summer months. For girls, no significant effects on handedness were observed, for birth order, maternal age, or season of birth.