Semen is a complex mixture of different compounds, and sperm actually only makes up a small amount of it. When you remove the sperm, what's left is seminal plasma, a fluid that contains an array of ingredients, some of which can pass through the vagina and be detected in the bloodstream after sex. Three compounds of interest in seminal plasma are estrogen, prostaglandins and oxytocin. Estrogen and prostaglandins have been linked to lower levels of depression, while oxytocin (which women release during birth, breastfeeding and orgasm) promotes social bonding. These and other compounds in semen could function to keep women coming back for more. "I think there's reason to believe based on some of the evidence we've collected that females that are in committed relationships that are having unprotected sex may use sex in part to self-medicate," Gallup says. "It's discovered after the fact that being inseminated has effects on mood, and they use sex to modulate their mood."