An imbalanced gender ratio affects views about casual sex for both men and women in ways that people may not consciously realise, a new study says.

More women than men on college campuses may contribute to a hook-up culture, engage in casual sex and are more aggressive towards other desirable women who are perceived as rivals.

"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 from Florida State University in the US.

"You might also have to cater to the demands of the other sex more often," Moss added.

In first 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.

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 unfavourable.

Participants who believed there was an unfavourable gender ratio were more likely to display unprovoked aggression with longer and louder noise blasts against attractive partners.

The same effects were not seen for unattractive partners, possibly because they were not seen as a threat. When the gender ratio was favourable for participants, they were less aggressive toward attractive competitors.

The research was published online in the journal Personality and Social Psychology Bulletin.