Mothers prefer to hold their babies in their left arm in nearly all cultures, as shown by contemporary studies and studies of artwork throughout history. The current explanation is that there exists an instinctive attempt to recreate the fetal habitat by placing the child's head near to where the maternal heartbeat can be heard. An additional explanation is that the left-sided preferences is a manifestation of hemispheric dominance, since the right hemisphere is dominant for affective and attentive processes, thus making the left side the site of maximal attention. However, the intensity of left- sided preference appears to have change throughout historical time, as shown by studies of artwork; this ought not to be the case if the heartbeat theory alone held true, given that this is a constant. It is postulated here that hemispheric bias can shift from one side to another, and examples are given that are socioeconomic, linguistic, and of sexual orientation in nature might explain why at certain periods of historical time, the left- sided preference was stronger than in others. In this light, 71 pieces of pre-Columbian ceramics were studied; 37 were Mesoamerican (Mexico and Northern Central America) and 34 were Andean (Peru, Ecuador, and Columbia). Overall left-sided preference was 65%, without significant difference between the two cultures. However, the period roughly encompasses between 300 b.c. and 600 a.d. (accounting for 45 pieces—23 Mesoamerican and 22 Andean) and gives 83% left-side preference for Mesoamerica as against 54% for the Andean area, which is significantly different ( p < 0.05). That particular period of history had distinctive cultural characteristics; it is taken as an example of how left-sided preference can be quantitatively different in one society as opposed to another. If this hypothesis is true, hemispheric bias in the only likely explanation.