Birth order and hand preference in chimpanzees (Pan troglodytes): Implications for pathological models of handedness in humans

The effect of birth order on hand preference was assessed in a sample of 15 4 captive-born chimpanzees. Subjects were classified as first, middle, or l atter born using 2 classification criteria based on their birth order. Hand preference was measured using a task that elicited coordinated bimanual ac tions. Significant birth-order effects were found for both classification c riteria, with first- and latter-born subjects exhibiting a lesser degree of right-handedness compared with middle-born subjects. These data suggest th at biological rather than sociological factors play a greater role in expla ining the observed birth-order effects on hand preference in humans.