Some prefer to simply follow an individual's self-definition or identity. However, most definitions include a psychological component (such as the direction of an individual's erotic desire) and/or a behavioural component (which focuses on the sex of the individual's sexual partner/s). Much of the research about sexual orientation has failed to define the term at all, making it difficult to reconcile the results of different studies. A number of different classification schemes have been used to describe sexual orientation since the mid-19th century. The definition of sexual orientation has changed greatly over time and the word 'gay' wasn't used to describe sexual orientation until the 20th century. Famous people who are simply rumored to be gay, lesbian or bisexual, are not listed, but since it is difficult to provide proof of a historical person's sexuality, they may be included where there are reliable sources. This is a partial list of famous people who were or identify themselves as gay, lesbian or bisexual. You can help by adding missing items with reliable sources. This is a dynamic list and may never be able to satisfy particular standards for completeness.