Gender psychology is the study of the behaviors and roles that are acquired by individuals under the premise that males and females will, in fact, behave in different ways because of their sexual roles as males or females.
However, aside from the study of the behaviors of males and females, gender psychology extends to the study of the behaviors of individuals who assume the roles and behaviors of a gender opposite to their own, for example, individuals who identify themselves as transgendered, bisexual, homosexual,or any other gender-based identity.
Gender psychology includes the gender schema theory as a topic of study. This theory states that nurture plays an influential factor in the sexual identification of boys and girls. According to this paradigm, boys and girls "learn" to act like boys and girls, respectively, as they learn those behaviors from other kids, and as they get molded by their parents to become "all boys" or "all girls".
However, what happens when nurture is not enough, no matter how strongly the gender-appropriate behaviors are imposed? What if a male child, raised by a masculine and heterosexual male, still grows up to discover that he has homosexual tendencies OR effeminate mannerisms: Who is to say that there is something wrong? Is it nature that influences our behaviors and gender-based mannerisms? Is it nurture? We will never know.
This is when gender psychology comes in with the latest research in human development and gender theory so that all those answers can be answered appropriately.