A new study claims to have identified the evolutionary driver behind so-called “duty sex” — intimacy pursued out of obligation rather than desire — and the answer isn’t relationship length or gender. It’s fear that a partner will cheat.
The research, published in the journal Evolutionary Psychology, surveyed 252 people between the ages of 18 and 24, including 176 women and 76 men. Most identified as heterosexual, and relationships among participants ranged from just starting out to as long as 84 months.
Lead author F. Sid Dougan set out to answer a specific question: why would obligatory sex persist as a behavior across human evolutionary history if it isn’t driven by desire. “What reproductive advantages might this behavior have provided over our evolutionary history? I wanted to know if duty sex serves an ultimate function, and if so, what that function is,” Dougan said.
Participants filled out surveys covering age, gender, race and ethnicity, education, sexual orientation, relationship status, number of lifetime partners, and current sexual activity, including vaginal, anal, or oral sex. They also answered questions measuring how often they would have sex to protect a relationship, and researchers measured three factors to predict that behavior: perceived probability of a partner’s infidelity, mate value discrepancy, and neuroticism.
The results: men and women engaged in duty sex at equal rates when they feared their partner might cheat on them. Fear of infidelity outranked every other factor tested as a predictor of duty sex.
That finding tracks with earlier research showing couples ramp up “mate retention” behaviors when they sense a higher risk of infidelity — during rough patches, for instance, or stretches of separation. Researchers believe duty sex under those conditions may function as an instinctive, evolved strategy to keep a partner from leaving.
Two other findings stood out. How long a couple had been together made no difference in how often either men or women engaged in duty sex. And despite prior research suggesting men generally have higher sex drives, men and women in this study reported participating in duty sex at the same rate.
The study authors flagged real limitations: the data was self-reported, the sample size was small, and participants skewed young and were mostly in the early stages of their relationships.
Still, the authors argue the findings push the conversation about duty sex in a new direction. “This work highlights how behaviors that appear obligation-based may reflect strategic responses to perceived partner loss risk, offering a foundation for future research on the role of sexual behavior in relationship maintenance,” they wrote.
