The greater proportion of women than men on college campuses may contribute to a hook-up culture where women are more willing to engage in casual sex and are more aggressive toward other desirable women who are perceived as rivals, according to new research published by the Society for Personality and Social Psychology.

In the first experimental study to examine this issue, researchers found an imbalanced gender ratio affects views about casual sex for both men and women in ways that people may not consciously realize.

"If your gender is in the majority, then you have to compete with a lot of rivals, and you can't be as selective or choosy," said lead researcher Justin Moss, an adjunct psychology professor at Florida State University. "You might also have to cater to the demands of the other sex more often."

The gender ratio at U.S. colleges has become more skewed over the past decade as more women attend college and graduate at higher rates than men, who are more likely to drop out. Last year, 57 percent of college students in the United States were women, according to the National Center for Education Statistics, with the gender ratio even more imbalanced at some schools. The study findings could have important practical implications about risky sexual behavior at colleges and in other areas, including efforts to reduce teen pregnancy rates and workplace sexual harassment, Moss said.

In one experiment, 129 heterosexual university students (82 women, 47 men) read one of two fake news articles stating that colleges in the local surrounding area were becoming either more female-prevalent or male-prevalent. The participants then completed a survey about their attitudes toward casual sex and their prior sexual history. The research was published online in the journal Personality and Social Psychology Bulletin.

When the gender ratio was favorable (one's own gender was in the minority), both men and women adopted more traditional sexual roles with women less interested in casual sex than men, according to the study findings. When the gender ratio was unfavorable (one's own gender was in the majority), those roles shifted as men and women tried to appear more desirable to the opposite sex. If there were more women than men, women stated they were more willing to engage in casual sex. If there were more men than women, men tended to place less importance on casual sex and be more open to long-term commitment.

In another experiment with 177 university students (73 women, 104 men), both men and women were more willing to deliver painful sound blasts to attractive same-sex competitors when the gender ratio was unfavorable. After participants read either the male-prevalent or female-prevalent article from the first experiment, they were told they would be competing on a time-reaction task against a same-sex partner in another room, although there was no real partner and the participants' responses were recorded by the researchers. One group was shown a picture of an attractive competitor who was described as an outgoing, sociable student, while the other group saw a photo of a less attractive competitor who didn't go out much and played a lot of video games.

In the time-reaction task, the participants were told to hit a computer key as soon as they heard a tone played through some headphones. When participants lost, they heard a painfully loud noise blast. When they won, they got to choose the length and volume of the noise blast that ostensibly would be inflicted on the competitor.

Participants who believed there was an unfavorable gender ratio were more likely to display unprovoked aggression with longer and louder noise blasts against attractive partners. The same effects weren't seen for unattractive partners, possibly because they weren't seen as a threat. When the gender ratio was favorable for participants, they were less aggressive toward attractive competitors.

The study participants were heterosexual so the findings don't necessarily apply to gay men or lesbian women. College campuses often have insular dating scenes so the research may not be as directly relevant to the general population where gender ratios are less skewed. However, Moss believes the same effects may be seen in other areas with imbalanced gender ratios, such as high schools or workplaces that are predominantly male or female, and even in smaller environments like bars.

"If a woman goes to a bar and notices a lot more women and thinks she has to compete, maybe she can consciously alter the course of her actions or leave and go to a different bar," Moss said. "Someone's personal views toward casual sex play an important role, but there also are environmental factors that people should consider."