From symmetry to asymmetry? The development of smile

The left side of the face has been reported to be more expressive than the right side in human adults. The developmental origin of this lateralization, however, is unknown. Chimeric still composite photos of smiling faces of 0–5 months (n = 61), 6–12 months (n = 67) old infants, 3–8 years old children (n = 78), and 32 adults (>18 years) were rated for intensity by 90 judges. The results showed left-hemiface dominance in adults only, and not in infants or children. It can be speculated that this early functional symmetry may evolutionarily increase attractiveness and elicits secure bonding and care.