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). Indeed, several studies have found that 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, and scholars have often defined the term 'sexual orientation' in divergent ways. The historical concept and definition of sexual orientation varies and has changed greatly over time for example the word 'gay' was not used to describe sexual orientation until the mid-20th century.
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