When presented with each of these three pairs of stimuli, newborns look longer at the image in the left–hand column, revealing a general preference for top–heavy stimuli that contributes to their preference for human faces (Macchi Cassia et al., 2004; Simion et al., 2002). Notice that this simple preference is all that is needed to result in newborns spending more time looking at their mother's face than at anything else. By 3 months of age, however, infants no longer discriminate between the faces in the middle pair of pictures, suggesting that their visual attention is no longer guided by a general top–heavy bias (Macchi Cassia et al., 2006).