Alfred Kinsey interviewed around 18,000 people in the 1940s and 50s, at a time when nobody had asked in any systematic way, and the work was genuinely foundational. His scale from 0 to 6 was also the first widely used model to treat orientation as a spectrum rather than two boxes.
The sampling, though, was not random. He recruited through groups he had access to, which included a substantial number of prison inmates and sex workers. The often-quoted “10 per cent are homosexual” was a reading of that sample. Modern probability samples put the figure for exclusively same-sex attraction at around 2 to 4 per cent, with far more reporting some.
