Ballistic Timing of Smiles is Robust to Context, Gender, Ethnicity, and National Differences

Smiles are highly variable. In some, contraction of the orbicularis oculi raises the cheeks and amplifies their intensity. In others, smile controls counteract the oblique pull of the zygomatic major, alter their shape, and decrease their intensity. Despite this variability, some features appear to be stereotypic. These features include a high correlation between the amplitude and velocity of smile onsets and same for smile offsets. The larger a smile's amplitude, the greater its velocity. This dependence is referred to as ballistic timing. In two relatively large publicly available databases (EB+ and Belfast), we tested the hypothesis of ballistic timing of smile onsets and offsets. We found high and consistent non-linear correlations between amplitude and velocity of both onsets and offsets that were robust to individual differences in persons (gender and ethnicity), context, presence or absence of the Duchenne marker, and country of residence (United States, Ireland, Peru). All $R_{2}$ exceeded 0.85. The findings were highly consistent with ballistic timing. They have implications for detecting smiles that may be posed (which have been found to violate ballistic timing) and for realistic synthesis of smiles in social robots and virtual humans. Smiles that depart from ballistic timing are likely to be perceived as false or uncanny.