The definition of pansexuality can encourage the belief that it is the only sexual identity that covers individuals who do not cleanly fit into the categories of male or man, or female or woman. However, bisexual-identified people and scholars may object to the notion that bisexuality means sexual attraction to only two genders, arguing that since "bisexual" is not simply about attraction to two sexes and encompasses attraction to different genders as well, it includes attraction to more than two genders. Gender is considered more complex than the state of one's sex, as gender includes genetic, hormonal, environmental and social factors. Furthermore, the term "bisexual" is sometimes defined as the romantic or sexual attraction to multiple genders. The Bisexual Resource Center, for example, defines bisexuality as "an umbrella term for people who recognize and honor their potential for sexual and emotional attraction to more than one gender", while the American Institute of Bisexuality states that the term "bisexual" "is an open and inclusive term for many kinds of people with same-sex and different-sex attractions" and that "the scientific classification "bisexual" only addresses the physical, biological sex of the people involved, not the gender-presentation." According to the National Center for Transgender Equality, 25% of American transgender people identify as bisexual.