A woman’s smile means warmth, a man’s mean confidence:
We present evidence that smiling is positively associated with positive affect in women and negatively associated with negative affect in men. In line with Vigil’s model, we propose that, in women, smiling signals warmth (trustworthiness cues), which attracts fewer and more intimate relationships, whereas in men, smiling signals confidence and lack of self-doubt (capacity cues), which attracts numerous, less-intimate relationships.
Source: Simine Vazire, Laura P. Naumann, Peter J. Rentfrow and Samuel D. Gosling (2009). Smiling reflects different emotions in men and women. Behavioral and Brain Sciences, 32 , pp 403-405