In Chapter 10 of the Lilienfeld text, there is a very brief section in which the textbook discusses gender identity, and the biological and social influences on gender. As defined by the book, gender identity is an "Individual's sense of being male or female." (pg. 392) It continues on to say that people tend to flock to objects and other people that are more closely related to the gender that they identify with, and that society tends to reinforce gender roles and stereotypes.
The way that the book describes this though is rather limiting, constricting to the gender binary, making gender rather black and white. The Lilienfeld text explains gender in too basic of a way, even for an Introduction to Psychology textbook. As someone who has spent several years reading about gender identity, and being involved in the transgender community, it is important to point out that gender is a lot more complicated than that, and it should be explained in depth.
Let us start by redefining gender Identity. According to University of California, Davis, gender identity is
"A person's internal sense or self-conceptualization of their own gender. Used to call attention to the self-identification inherent in gender."
To further explain, there are many more genders than just male and female. There is in fact a whole spectrum in which many people belong. I know of people who identify as male, female, genderqueer, omnigendered, transgender, two spirit, and so on and so forth. Gender also does not equal sex nor sexual orientation. Your sex is based on your genitalia, whereas your gender is based on what you feel your gender is, and your sexual orientation is based on who you're attracted to.
Your gender expression also does not determine your gender identity. One should never assume that just because someone dresses, acts, and appears a certain way, it does not mean that they should be fit snugly into one category. If someone looks more masculine, does more masculine things (or at least deemed masculine by society), that does not always mean that they identify as male, and vice versa. This also points to what the Lilienfeld text said in terms of biological influences. While there is a strong basis for those to do things that more fit their gender identity, it still undermines the fact that not everyone fits into those sorts of molds.
The book also failed to mention people who are intersex,
"People who naturally (that is, without any medical interventions) develop primary and/or secondary sex characteristics that do not fit neatly into society's definitions of male or female."Those who are intersex are often forcibly assigned a gender at birth (often times the wrong one), who then later on in life transition to the gender that they feel more comfortable with.
The discussion could go on for ages because gender is most definitely not black and white, and is probably one of the most colorful things you could come across. Like an onion, gender has many layers. It takes time to understand, and to wrap your mind about it. It's complicated, and should never be talked about in any other way that isn't.
For those reading who need a better, and more detailed explanation of any terms, or of the gender spectrum, here is a creative, and very informative comic: https://roostertails.wordpress.com/2011/05/18/sneak-peek-queer-101/