Women who have more male friends indulge in a lot more carnal activity with their partners than couples where the female has fewer male friends, says a new study.
This is because men perceive male friends in the lives of their female partners as "sex rivals".
"This is human nature. We need to be reminded that our partner is valuable to us and desirable to others. This makes us keep working at maintaining relationship satisfaction," lead researcher Michael Pham from the Oakland University in the US, told Fusion.net.
"Finding that right balance of inducing your partner's jealousy will optimise your sex life," he added.
To reach this conclusion, the team analysed 393 men in committed relationships.
The participants were asked about their partner's attractiveness, how many male friends they perceived their female partner had, how attractive they think these other men find their partner and how many times they had sex with their partner in the last week.
Men who said their partners attracted a lot of male attention were associated with having more sex with them, the authors found.
"Men are sexually aroused by their partner when they estimate a greater likelihood of partner infidelity," Pham added.
The study appeared in the Journal of Comparative Psychology.