Research published in Evolutionary Psychological Science has found that men orgasm faster and ejaculate more semen when masturbating to unfamiliar women.

“Our findings are the first to demonstrate that men’s ejaculate behavior and composition change in response to a novel female stimulus,” the researchers wrote in their study, which was published in June.

In the study, the researchers examined the time to ejaculation, ejaculate volume, and number of motile sperm in 21 heterosexual men between the ages of 18 and 23 years old who watched seven sexually explicit films over the course of 15 days.

The men in the study watched six films depicting the same actress and actor, then watched a similar film with a new actress but the same actor.

The researchers found the time to ejaculation ranged between 4 and 21 minutes overall. There was no habituation effect — repeatedly viewing the same woman did not increase or decrease the time to ejaculation.

But men did ejaculate more quickly when viewing the seventh film, which included a new woman. In addition, ejaculate volume and total number of motile sperm in the ejaculate increased significantly when viewing the seventh film. “Men produced higher quality ejaculates when exposed to novel, rather than familiar, women,” the researchers wrote.

But why does ejaculate volume and total number of motile sperm increase for novel women? Men who produce higher quality sperm for unfamiliar women may have an evolutionary advantage, the researchers explained. The decrease in the time to ejaculation may make it easier for a cheater to copulate with another woman without his partner finding out.

In addition, evolution “would favor males who invest more in ejaculates transferred to novel females for two reasons: (i) Males may have already fertilized the egg(s) of (or have their sperm stored by) females with whom they have already mated; and (ii) novel females may be more likely to have mated recently with another male resulting in increased likelihood of sperm competition,” they wrote.