In the 1950s and 60s, Masters and Johnson studied sexual function in 700 people who agreed to have their sex or self-pleasure watched under close scientific scrutiny.
The researchers measured bodily responses and studied the vagina during orgasm by placing a camera in a clear plastic penis.
One important contribution was defining the four stage model of sexual response, which they described as the human sexual response cycle defined as:
(1) Excitement phase (initial arousal)
(2) Plateau phase (at full arousal, but not yet at orgasm)
(3) Resolution phase (after orgasm)
Their findings, particularly female sexual arousal and orgasm, dispelled many long standing misconception and brought attention to sexual dysfunction.