Lawrence Kohlberg, an American psychologist, theory of gender identity in children believes that young children have yet to grasp that gender is a fixed concept. Three and four year old children often believe that something as simple as a short haircut will turn a girl into a boy. As adults, we know this is not true, but children have yet to understand that gender is fixed, and her thoughts and beliefs are not a sign that she will become bisexual or homosexual later in life.
Mom remain patient and understanding when your daughter makes these statements. Don't allow a possible fear of your child's sexual orientation changing, to dictate how you respond.
Mom, I ask that you look within yourself and ask yourself why you have these concerns? Are your concerns predicated by your religious beliefs? Would you be ashamed or embarrassed if your daughter chose to live as a boy?
Ask your daughter why she believes that she is a boy? Ask your daughter why she wants to be a boy. Explain to your daughter, in simple terms, what it means to be a boy and what it means to be a girl. The next time that she tells you that she is a boy, simply say, "Okay, and I am your mom".