Socio-emotional behaviour is in part sex-related in humans, although the contribution of the biological and socio-cultural factors is not yet known. This study explores sex-related differences during the earliest communicative exchange, the neonatal imitation in 43 newborn infants (3–96 hours old) using an index finger extension imitative gesture. Results showed that although the experimenter presented comparable stimuli to both sexes, and the total number of movements was similar in boys and girls, girls showed more fine motor movements, a higher number of specific imitative gestures, responded faster during the imitation and showed a higher baseline heart rate during the experiment. Newborn girls, with their faster and more accurate imitative abilities, may create a more responsive and interactive social environment, which in turn may lead to differences in socio-emotional and cognitive development between girls and boys. Copyright © 2007 John Wiley & Sons, Ltd.