Ear asymmetry and left-side cradling

Women and girls tend to cradle infants and dolls on the left side of the body. Left-sided cradling is found in chimpanzees and gorillas, is cross-cultural and present in historical works of art, and is transmitted down the human maternal line. One explanation for the left-cradling tendency is that it facilitates the flow of affective information from the infant via the left ear and eye to the center for emotional decoding, that is, the right hemisphere of the mother. We show that the developmental stability of the ear, as measured by ear asymmetry, is negatively correlated with the left-sided cradling tendency. Left-cradling English women holding infants and Jamaican girls holding dolls had a strong tendency to show lower ear asymmetry than right cradlers, whereas no such relationship was found in boys nor for various measures of asymmetry of the hand, with the possible exception of the wrist in Jamaican girls. Degree of handedness, as measured by the Annett peg-moving test, did not predict cradling preference in the Jamaican children, and the relationship between ear asymmetry and cradling preference was independent of hand preference. Our results suggest that developmental instability of the ear (including the pinna, external auditory meatus, and middle ear) may interfere with the flow of affective information to the right hemisphere. Ear asymmetry also showed evidence of strong maternal but not paternal transmission. It is suggested that between-individual variation of in utero stress may explain patterns of maternal transmission of lateral cradling tendencies.