Science of the Smile

Neurobiologist Margaret Livingstone has argued that the smile looks stronger in your peripheral vision than when you stare straight at it. When you focus directly on the mouth, the smile flattens. Look at her eyes, and it returns. Her explanation is that peripheral vision is tuned to soft, low-detail shapes and shadow, while central vision picks out fine detail — so the shadowed smile reads differently depending on where you look.

Leonardo almost certainly understood this effect intuitively. By dissolving the corners of the mouth into shadow, he left the expression deliberately ambiguous — so your brain fills in different feelings depending on your mood, your viewing angle, and where on the face your eye lands.