Recently, I've been seeing videos and news that say that gender identity is formed much earlier than commonly believed, with respect to gender identity disorders or being transgender. Such information has also been included in certain answers on this site.
However, it doesn't appear to have been explicitly asked here.
At what age or stage of development does a child's gender identity form?
To clarify the term, here's the first paragraph from the Wikipedia article linked to above.
Gender identity is a person's private sense and subjective experience of their own gender. This is generally described as one's private sense of being a man or a woman, consisting primarily of the acceptance of membership into a category of people: male or female. All societies have a set of gender categories that can serve as the basis of the formation of a social identity in relation to other members of society. In most societies, there is a basic division between gender attributes assigned to males and females. In all societies, however, some individuals do not identify with some (or all) of the aspects of gender that are assigned to their biological sex.
I'm not asking about:
- Gender roles
- Sexual identity
I am looking for research-backed answers only. Since what I perceive to be commonly-held beliefs (or concepts that aren't often thought about) don't match up with some of the medical opinions I've read, I would like evidence to help turn my loose ideas into strong understanding.