Fritz Klein, 73, a clinical psychiatrist who pioneered bisexual research, died May 24 of a heart attack at his San Diego home, the American Institute of Bisexuality announced.

Klein, an activist for the understanding of bisexuality who founded the institute and served as chairman of the board, had recently been diagnosed with cancer.

The Klein Sexual Orientation Grid expanded on the Kinsey scale of human sexuality in an attempt to accommodate what Klein saw as the complex and changing nature of a subject’s tendencies.

Klein studied patients’ lifestyles in addition to asking them to rate their behaviors and fantasies. He published his findings in “The Bisexual Option,” one of the first serious examinations of bisexuality as a separate sexual orientation.

Born in Vienna on Dec. 27, 1932, Klein moved with his family to New York as a child but returned to Europe for his medical education at the University of Bern. He also had a master’s degree in business administration from Columbia University.