That can’t be right! Well, it certainly doesn’t feel right. Whenever I tell this to people, they respond with dramatic incredulity. The 1901 Dorland’s Medical Dictionary defined heterosexuality as an “abnormal or perverted appetite toward the opposite sex.” More than two decades later, in 1923, Merriam Webster’s dictionary similarly defined it as “morbid sexual passion for one of the opposite sex.” It wasn’t until 1934 that heterosexuality was graced with the meaning we’re familiar with today: “manifestation of sexual passion for one of the opposite sex normal sexuality.” This story is featured in BBC Future’s “Best of 2017” collection.