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 wasn't used to describe sexual orientation until the mid-20th century. 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.
