RIO DE JANEIRO, BRAZIL - JUNE 28: A couple celebrates on Copacabana Beach following Brazil's World Cup match win over Chile on June 28, 2014 in Rio de Janeiro, Brazil. Host country Brazil advances to the quarter-finals. (Photo by Mario Tama/Getty Images) (Photo by Mario Tama/Getty Images)

WASHINGTON (CBS DC) — Women who hang out with more male friends and co-workers have more frequent sex with their boyfriends than women who have fewer male influences in their life.

The study — published in the Journal of Comparative Psychology — finds that men often perceive their female partner’s abundance of male friends to be a form of “sperm competition psychology,” in which the men have more sex with their girlfriends due to the potential of “sex rivals.”

The Oakland University study of 393 men in committed, sexual, heterosexual relationships found that human males – just as in nonhuman males – have more sex with their partner when they perceive that their female partner has an excess of male friends posing a “sperm competition risk.” This was made more evident when men perceived that their female partner was considered sexually attractive to the male “sex rivals” in her social circles.

“The results indicate that men whose in-pair partner has more male coworkers and friends (i.e., potential sexual rivals) also perform more frequent in-pair copulations, but only among men who perceive their partner to be particularly attractive relative to assessments of partners by other men in the sample,” write the researchers.

The researchers note that the biological urge for sperm competition is not an unhealthy element of a relationship and is instead simply a subconscious part of “human nature.”

“A lot of work shows that men are sexually aroused by their partner when they estimate a greater likelihood of partner infidelity,” Michael Pham, lead author of the study, explained to Fusion via email. “This does not mean that men want their partner to cheat on them. Rather, this means that if they estimate a greater likelihood of their partner’s infidelity, then they may (subconsciously) want to have sex with her to enter into sperm competition.”

“This is human nature,” Pham told Fusion. “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.” Adding, “Finding that right balance of inducing your partner’s jealousy will optimize a couple’s sex life.”

The study states that porn featuring two men having sex with one woman causes men to release more sperm due to a sense of “sex rivals” in the environment.

The nearly 400 couples studied had an average relationship length of 35.9 months and the men were asked to report on a series of issues including their female partner’s attractiveness, number of her male friends and details of their own sex life.

Todd K. Shackelford, co-author of the study, told The Daily Mail that the study results show men are competitive with other men by nature – even if the threat is just perceived.

“They [the women] don’t even have to see them having sex with other males. It’s just the presence,” he noted.

— Benjamin Fearnow