Dr. Fritz Klein, a psychiatrist who studied human sexuality and helped launch a foundation focused on bisexuality, died May 24 at his home in San Diego. He was 73.

Klein founded the American Institute of Bisexuality (AIB) and served as board chair. His life work was defined by his belief that sexual orientation is fluid and changes throughout a person’s life. Klein believed the number of men who were sexually active with both sexes had been undercounted and unrecognized by the scale developed by Alfred C. Kinsey in the 1940s.

Klein is best known for his pioneering sex research and the development of the multi-dimensional Klein Sexual Orientation Grid, which measures the complexity and fluidity of sexual orientation.

The Klein Grid, first published in 1978, expanded on the “zero to six” Kinsey scale. The Klein Grid measures actual sexual experiences, but also sexual attractions, fantasies, emotional preference, social preference, lifestyle and self-identification as they relate to a person’s past, present and ideal future. Klein’s research showed that these factors can change over time for an individual, and vary not just between but also within groups of straight, gay and bisexual people. He concluded that people generalize from their own experiences and feelings to assume, often wrongly, that other people must experience their own sexual orientations the same way.

As a result, Klein concluded sexual orientations are too complex to be broken into simple, well-defined categories.

Nonetheless, he was a tireless activist, especially concerning bisexual issues and community. He was known all over the world for his groundbreaking research and writing, ranging from the academic publication of the Klein Grid in the Journal of Homosexuality in 1985 to popular books and articles concerning bisexuality, as well as a novel, Life, Sex and the Pursuit of Happiness, published in 2005 by Harrington Park Press.

Klein was born Fred Klein in Vienna, Austria, in 1932. While still a small boy, he fled with his family to New York City to escape anti-Semitism and the impending war. He later studied medicine in Switzerland at Bern University and received an MBA from Columbia University. Klein was a board-certified psychiatrist for 30 years in New York and San Diego until his retirement.

Early in his career, he realized that there was a void in knowledge about sexual orientation, specifically in the area of bisexuality. He placed an ad in the Village Voice for a meeting that resulted in the creation in New York of the Bisexual Forum, which recently celebrated its 20th anniversary. During this period, he wrote The Bisexual Option (1978) and co-authored Man, His Body, His Sex (1978).

After moving to San Diego, Klein founded the Bisexual Forum in this city in 1982, wrote several books, and founded and became editor of The Journal of Bisexuality.

Klein founded the American Institute of Bisexuality, a public benefit charity, in 1998, to encourage, support and assist research and education about bisexuality, and served as chair of the board up until his death. He was known for being outspoken, controversial and compassionate, and for his love and support of theater and the arts.

Though he had recently been diagnosed with cancer, his death from cardiac arrest was sudden and unexpected. He is survived by his life partner, Tom Reise of San Diego, and two brothers, George and Seymour of New York City.

Klein chose to donate his body to science. A celebration of his life is being planned. In lieu of flowers, contributions may be made to San Diego’s Diversionary Theatre.

The American Institute of Bisexuality, also known as the Bisexual Foundation, encourages, supports and assists research and education about bisexuality through programs likely to make a material difference and enhance public knowledge and awareness.

Press release