^Sexual Orientation and Gender Expression in Social Work Practice, edited by Deana F. Morrow and Lori Messinger (2006, ISBN0231501862), page 8: "Gender identity refers to an individual's personal sense of identity as masculine or feminine, or some combination thereof."
^V. M. Moghadam, Patriarchy and the politics of gender in modernising societies, in International Sociology, 1992: "All societies have gender systems."
^Carlson, Neil R.; Heth, C. Donald, Sensation, (编) Carlson, Neil R.; Heth, C. Donald, Psychology: the science of behaviour 4th, Toronto, Canada: Pearson: 140–141, 2009, ISBN 9780205645244.
^Jack David Eller, Culture and Diversity in the United States (2015, ISBN1317575784), page 137: "most Western societies, including the United States, traditionally operate with a binary notion of sex/gender"
^For example, "transvestites [who do not identify with the dress assigned to their sex] existed in almost all societies." (G. O. MacKenzie, Transgender Nation (1994, ISBN0879725966), page 43.) — "There are records of males and females crossing over throughout history and in virtually every culture. It is simply a naturally occurring part of all societies." (Charles Zastrow, Introduction to Social Work and Social Welfare: Empowering People (2013, ISBN128554580X), page 234, quoting the North Alabama Gender Center.)
^A few authorities say it forms between ages 3-4 rather than precisely at age 3, e.g. George J. Bryjak and Michael P. Soraka, Sociology: Cultural Diversity in a Changing World (ed. Karen Hanson), Allyn & Bacon, 1997; 209-245
^Christopher Bates Doob, Social Inequality and Social Stratification in US Society
^ 14.014.114.2J. A. Kleeman, The establishment of core gender identity in normal girls. I.(a) Introduction;(b) Development of the ego capacity to differentiate, in the Archives of Sexual Behavior, 1971: "Though gender identity formation continues into young adulthood and core gender identity establishment extends into the fourth year and possibly longer, core gender identity is fairly firmly formed by age 3[.]"
^E. Coleman, Developmental stages of the coming out process, in Journal of homosexuality, 1982: "Core gender and sex-role identities are well-formed by the age of 3 (Money & Ehrhardt, 1972). This is believed because attempts to reassign gender identity after age 3 result in further gender dysphoria."
^Stein MT, Zucker KJ, Dixon SD. December, 1997. "Gender Identity", The Nurse Practitioner. Vo. 22, No. 12, P. 104
^Martin, C.; Ruble, D. Children's Search for Gender Cues Cognitive Perspectives on Gender Development. Current Directions in Psychological Science. 2004, 13 (2): 67–70. doi:10.1111/j.0963-7214.2004.00276.x.
^ 20.020.1Effects of male sex hormones on gender identity, sexual behavior, and cognitive function, Zhong Nan Da Xue Xue Bao, Yi Xue Ban (Journal of Central South University, Medical Sciences), April 2006, 31(2):149-61
^Susan Golombok, Robyn Fivush, Gender Development (1994, ISBN0521408628), page 44: "When assigned and raised as boys, these genetic girls adopt a male gender identity and role, showing that a Y chromosome is not necessary for gender development to proceed in a male direction."
^Henslin, James M. Essentials of Sociology. Taylor & Francis. 2001: 65–67, 240. ISBN 0-536-94185-8.
^Lynda Birke suggests that during the early stage of fetal development, specific hormones will enter the brain and “permanently affect how the hypothalamus works. As before, high levels of hormones known as androgens will stop the hypothalamus from ever organizing hormone cycles. If there are low levels, then it will be cyclic.” This early influence on brain determines the different frequency of hormone secretion later in male or female’s life. “Obviously, women’s sex hormones usually follow a monthly cycle,” while “men’s sex hormones do not follow such a pattern.” Birke, Lynda. The Gender and Science Reader. : 313.
^Meyer-Bahlburg, HF. Gender identity outcome in female-raised 46,XY persons with penile agenesis, cloacal exstrophy of the bladder, or penile ablation.. Archives of Sexual Behavior. PMID 16010465. doi:10.1007/s10508-005-4342-9.
^Anne Fausto-Sterling, Sexing the Body: Gender Politics and the Construct
^D. F. Swaab, Sexual differentiation of the human brain: relevance for gender identity, transsexualism and sexual orientation, in Gynecological Endocrinology, 2004: "...direct effects of testosterone on the developing fetal brain are of major importance for the development of male gender identity and male heterosexual orientation. Solid evidence for the importance of postnatal social factors is lacking."
^M. S. C. Wallien, Psychosexual outcome of gender-dysphoric children (2008)
^M Weinraub, LP Clemens, A Sockloff, T Ethridge, The development of sex role stereotypes in the third year: relationships to gender labeling, gender identity, sex-types toy preference, and family characteristics, in Child Development, 1984: "Previous investigators have failed to observe a relationship between parental attitudes and children's early sex role acquisition..."