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. A number of different classification schemes have been used to describe sexual orientation since the midth century, and scholars have often defined the term "sexual orientation" in divergent ways. 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. Some prefer to simply follow an individual's self-definition or identity. The high prevalence of people from the West on this list may be due to societal attitudes towards homosexuality. But there is far greater tolerance for homosexuality in major Latin American countries such as Mexico, Argentina, Bolivia and Brazil. Opinion in Europe is split between West and East.