The Kinsey Scale is a scale which is designed to represent the diversity and gradation of human sexual orientation, even within an individual person's life history. It was devised by notable researcher Alfred Kinsey, who realized that human sexual orientation fluctuates throughout life, and that it rarely falls into simple black and white categories. The development of the Kinsey Scale allowed people to appreciate the diversity of human nature more fully, encouraging a more open-minded attitude to human sexuality.
On the Kinsey Scale, people fall between a one and a six. Someone who measures as a one is exclusively heterosexual, while someone who measures a six is exclusively homosexual. The gradations in between take a varying range of experiences into account, emphasizing the idea that people's natures are often fluid and flexible. In the Kinsey Report, Kinsey's landmark work on human sexuality, another category was included for people who identify as asexual; asexuals are categorized as “X,” reflecting the fact that they fall outside the scale.
Over the course of Kinsey's research, Kinsey and his assistants interviewed thousands of people, and made a number of interesting discoveries, including the fact that someone's place on the Kinsey Scale often changes over the course of his or lifetime, as he or she grows and matures. His research included a comprehensive breakdown of categories, looking at differences between married and single people, men and women, different socioeconomic classes, and different races.
The widespread publication of the Kinsey Scale helped to dispel the idea that people cleave firmly to one sexual orientation throughout their lives, and it also helped to break down social conceptions around homosexuality. Around 10% of white males in the study between 20 and 35, for example, fell on the scale as threes. Kinsey's research was quite groundbreaking for the late 1940s and early 1950s, when his work was published, and it generated a fair amount of controversy, not least because of the inclusion of the Kinsey Scale.
As often happens with statistics, a number of people have tried to use the Kinsey Scale to prove and disprove various things. Some commentators attempted to put words in Kinsey's mouth, or to manipulate the scale in ways which satisfied their own ends. The goal of the Kinsey Scale was not to make a formal statement about human nature, but simply to open people's eyes, illustrating the fact that humans tend to strongly resist being categorized.