What is gender?
Gender is commonly defined in terms of biology and anatomy. Although this may be more accurately called one's sex - male, female, intersex. One's gender also connotes the societal expectations associated with each sex - male, female, transgender.
What about gender identity?
"Gender identity is how we feel about and express our gender and gender roles — clothing, behavior, and personal appearance. It is a feeling that we have as early as age two or three."
[Planned Parenthood. Health Info and Serivces. Gender and Gender Identity (What is Gender? What is Gender Identity?) http://www.plannedparenthood.org/health-topics/sexual-orientation-gender/gender-gender-identity-26530.htm]
Don't forget the databases on the Welcome page. Here are a few specific to Gender.