Gender psychology is the study of the traits or characteristics that are associated with the biological sex of subjects. The western world has generally and historically conceived gender as being either male or female, with strict definitions of masculine and feminine behavior. There are, however, cultures in the west and other parts of the world are that less rigid in their gender definitions and recognize additional gender labels. Starting in the mid 20th century western science started to openly recognize and study variations in human sexuality such as homosexuality, bisexuality, transsexuality, transvestism, and others.