Two studies, published in the Journal of Vision, found scowls and smiles are often considered cues for gender identification.
In the first study, androgynous faces with lowered eyebrows and tight lips -- categorized as angry expressions -- were more likely to be identified as male. Those with smiles and raised eyebrows -- expressions of happiness and fear -- were often labeled feminine.
In the second study, when male and female faces wearing expressions of happiness, anger, sadness, fear or a neutral expression were shown to study participants, the female faces that expressed anger took the longest to identify.
"The present research shows that the association between anger and men and happiness and women is so strong that it can influence the decisions about the gender of another person when that person is viewed briefly," study author Ursula Hess of the University of Quebec in Montreal said in a statement.