Psychologist and sexologist Virginia Johnson, who pioneered research on homosexuality, the stages of sexual arousal, and reproductive biology, died Wednesday at an assisted living center in St. Louis. She was 88.

Johnson and her ex-husband William H. Masters (who passed away in 2001 at the age of 85) formed the research team Masters and Johnson. The couple established the Masters and Johnson Institute in St. Louis in 1964, which was originally called the Reproductive Biology Research Foundation. At their institute, Masters and Johnson conducted large-scale human sexuality experiments.

One of the couple's landmark studies identified four stages of sexual arousal experienced by both men and women: the excitement phase, plateau phase, orgasmic phase, and resolution phase. The stages became known as the human sexual response cycle. According to the Associated Press, Masters and Johnson also changed perceptions about female sexuality, challenging Freud's concept of the female orgasm. Freud believed that vaginal orgasm was the "more mature sexual response for women." But Johnson's research found that clitoral stimulation and orgasm proved to be equally important.

Masters and Johnson were believed by many to have "revolutionized the way sex [was] studied," asking questions that many were afraid to ask and doing intricate experiments on sexual intercourse. The expertise that they developed from their research resulted in the couple becoming popular figures as sex experts in the media.

Masters and Johnson's findings were published and discussed in two books. One entitled Human Sexual Response was published in 1966. Another, in the 1970s, was called Human Sexual Inadequacy. Among Johnson's family members, the books that the couple published were their greatest accomplishments.

"The family feeling was they changed the study of sex with the landmark publishing of their books," said Scott Johnson, Johnson's son.

A television show called Masters of Sex, based on Johnson's romantic and professional relationship with Masters, is set to debut on Showtime in the fall. The one-hour series will chronicle their lives and "pop culture trajectory."