Debora MacKenzie in New Scientist:
Ben Jones at the University of Aberdeen in Scotland, and colleagues, showed 28 men and 28 women pairs of male faces and asked them to rate their attractiveness. The photos had been already been rated by 40 women as of about equal attractiveness.
The researchers then showed the same faces alongside a third photo of a female face in profile, positioned so she was looking at one of them, and smiling – or not. The viewers were asked to grade the faces again.
Women found the men who were being smiled at suddenly more attractive, while men who apparently elicited no such smiling approval were pronounced less attractive.
More here.