J. Dunkeld, The Function of Imitation in Infancy, Unpublished Ph.D. Thesis, University of Edingburgh, 1978

MALE INFANTS MAY HAVE DIFFERENT SMILES FROM FEMALE INFANTS

The origins of the different movement patterns are at present obscure. It is possible that parents reinforce particular patterns of movement in their infants. It is possible that the skeletal differences between boys and girls determine different patterns of movement.