Fred (Fritz) Klein (December 27, 1932 May 24, 2006) was an American sex researcher, psychiatrist, inventor of the Klein Sexual Orientation Grid and author. He was also a pioneering bisexual rights activist, who was an important figure in the modern LGBT rights movement.
As a self-identified bisexual, Klein was surprised at the lack of literature on his sexuality in the New York Public Library in 1974. He was inspired to place an advertisement in a New York City alternative newspaper the Village Voice and founded a ground-breaking social and support group for the Bisexual Community called Bisexual Forum.
He devised the Klein Sexual Orientation Grid, a multi-dimensional system for describing complex sexual orientation, similar to the "zero-to-six" scale Kinsey scale used by Alfred Kinsey, but measuring seven different vectors of sexual orientation and identity (sexual attractions, sexual behavior, sexual fantasies, emotional preference, social preference, lifestyle and self-identification) separately, as they relate to a person's past, present and ideal future.
Klein moved to San Diego in 1982. There he founded the American Institute of Bisexuality (AIB), also known as the Bisexual Foundation, in 1998 to encourage, support and assist research and education about bisexuality. Klein also founded the Journal of Bisexuality. He remained the Journal's principal editor until his death.
In 2006 Klein was diagnosed with cancer, and underwent surgery as a result. He died unexpectedly at home from cardiac arrest at age 73. He was survived by two brothers and his life partner, Mr. Tom Reise. He donated his body to science.