A lesbian is a homosexual woman. The concept of "lesbian" to differentiate women with a shared sexual orientation evolved in the 20th century. Throughout history, women have not had the same freedom or independence as men to pursue homosexual relationships, but neither have they met the same harsh punishment as homosexual men in some societies. Instead, lesbian relationships have often been regarded as harmless and incomparable to heterosexual ones, unless the participants attempted to assert privileges traditionally enjoyed by men. As a result, little in history was documented to give an accurate description of how female homosexuality was expressed. When early sexologists in the late 19th century began to categorize and describe homosexual behavior, hampered by a lack of knowledge about homosexuality or women's sexuality, they distinguished lesbians as women who did not adhere to female gender roles and incorrectly designated them mentally ill—a designation which has been reversed in the global scientific community.