In 1957 William Masters , a gynaecologist at the Washington University School of Medicine in St Louis, engaged Virginia Johnson to assist him in the research into human sexuality he had begun a few years earlier. They were to work together for more than three decades, publishing a range of landmark books and transforming attitudes to sex and sexuality.

Before Masters and Johnson did their research, sex was often shrouded in mystery and misconceptions, with discussion hampered by embarrassment and taboo. Alfred Kinsey and his reports on sexuality in the 40s and 50s had paved the way for Masters and Johnson. They in turn made possible the work of Alex Comfort and his 1972 sex manual The Joy of Sex , several generations of agony aunts and sex experts, and the acceptance of sex as a subject for dinner-party debate.

Johnson, who has died aged 88, is often described as a sexologist or psychologist. In fact, she had been studying for a sociology degree when Masters took her on, and the demands of their work together prevented her from completing it. She grew from being assistant to collaborator and an essential component of the partnership: Masters had academic credentials and research experience; Johnson had warmth, people skills and administrative ability. It was she who managed to get nurses, students and other volunteers to shed their clothes and inhibitions to take part in what became one of the largest human sexuality studies in the US.

She was born in Springfield, Missouri, daughter of Edna and Harry Eshelman. When Virginia was five, her family moved to Palo Alto, California, where her father worked as a hospital groundskeeper, but they eventually returned to Missouri and farming. Although at 16 Virginia enrolled at Drury College (now University) in Springfield, she left to work in the state insurance office for four years. A passion for music took her back to education ? at the University of Missouri and the Kansas City Conservatory of Music ? and she sang with a band during the second world war.

By her early 20s, she had married and divorced twice, and moved to St Louis. After her third marriage, to George Johnson, a bandleader, ended in divorce, she decided she no longer wanted to pursue a singing career, and enrolled at Washington State, where she met Masters. She had been a business writer for the St Louis Daily Record, and her secretarial and writing skills were perhaps what attracted him, but he got much more.

Masters had secured funding and permission to study human sexuality and had been observing prostitutes in local brothels ? hardly representative of the average, as one of the workers apparently pointed out. He decided he needed a female research partner to be able to reassure and thus attract volunteers to allow him to assess their physical reactions while they had sex or masturbated. Over a decade, Masters and Johnson studied 382 women and 312 men aged between 18 and 89, wiring them up and filming them to measure a range of reactions.

Written in deliberately dry and scientific language to allay accusations of being smutty, their first book, Human Sexual Response (1966), was nevertheless a sensation. Among other things, their research laid to rest Sigmund Freud 's idea that female orgasm was either clitoral (according to Freud, immature and thus bad) or vaginal (mature, given by men and thus good). The polygraph-like instruments Masters and Johnson designed and created showed that female orgasm, whether stimulated directly or indirectly, was clitoral.

They identified the four stages of sexual response ? excitement, plateau, orgasmic and resolution ? and showed that these occurred in both sexes. They demonstrated that the size of the male member is not linked to performance; that women can be multi-orgasmic; and that there is no time limit on sexual desire, as sex among elderly people is common and normal. Johnson's approachable manner and accessible language meant that she and Masters were in great demand to appear on chatshows and featured in magazines, including on the front cover of Time . Their book became a bestseller and words such as clitoris, orgasm and masturbation were suddenly out in the public domain.

Using what they had learned about the sexual response cycle, Masters and Johnson began offering therapy designed to help couples affected by sexual dysfunction. Instead of long-term psychoanalytical therapy, their suggestion was a short, focused course to help people recognise their own triggers, blocks and preferences. This became known as sensate focus: a way for couples to reconnect and for individuals to develop sexual self-awareness that is still the standard for sexual therapy. As Masters was to say: "Sex is a natural function. You can't make it happen, but you can teach people to let it happen." In 1970 they published their findings in Human Sexual Inadequacy.

Having been lovers from quite early in their collaboration, Johnson and Masters married in 1971. They had founded the Reproductive Biology Research Foundation in St Louis in 1964, and in 1978 renamed it the Masters and Johnson Institute.

Their sure touch went missing with two subsequent publications. Opponents of gay rights objected when Masters and Johnson, in Homosexuality in Perspective (1979), championed the right of gay men and women to have their sexual problems respected and treated; while others were appalled that the treatments they offered included "cures" for gay people. In Crisis: Heterosexual Behavior in the Age of Aids (1988, with Robert C Kolodny), their language and claims appeared alarmist and inaccurate.

Masters and Johnson divorced in 1993, although they continued to be friends and collaborators. The following year, Masters retired and closed the institute. He died in 2001. In the late 1990s Johnson opened the Virginia Johnson Masters Learning Center, in Creve Coeur, Missouri, providing advice on overcoming sexual dysfunction.

She is survived by two children, Scott and Lisa, and two grandchildren.