They say variety is the spice of life, and provocative new research by scientists in Ohio suggests that that holds true in the world of human sex and reproduction.
With the help of 21 men and some porn videos, The College of Wooster researchers showed that guys ejaculate faster and produce more, higher-quality sperm when they masturbate to a "novel female stimulus."
In other words, they really get off with a hot woman they've never previously encountered.
The finding doesn't mean that men are somehow programmed to cheat on their girlfriends and wives. It may suggest that men have evolved such that it's easier for them to have sex with (and impregnate) other women without their usual partners finding out.
As study co-author Dr. Laura K. Sirot, assistant professor of biology at the college, told The Huffington Post in an email, "Our study does not suggest anything about whether monogamy was favored or disfavored by natural selection. However, the results of this study and other studies of human differential ejaculation behavior suggest that extra-pair (or extramarital) copulations may have occurred in our evolutionary past."
In addition to providing new insights into evolutionary biology, the finding may lead to better detection and treatment of male infertility.
When men see a doctor about fertility problems, they often provide semen samples collected while masturbating to porn. And since it features novel women, "the ejaculates produced in fertility clinics may be of higher quality than usual, which may conceal any potential fertility problems experienced in the bedroom," study co-author Paul N. Joseph, a 2014 graduate of the college, said in a written statement.
So the finding may help spare women from invasive diagnostic procedures to spot fertility problems that really lie with their partners.
For the study, the researchers analyzed semen samples collected by 21 heterosexual men as they masturbated to seven sexually explicit films from the "Intimate Passions" series over a 15-day period. The first six films featured the same actress and actor, the seventh the same actor but a different actress.
The men were asked to abstain from alcohol and sexual activity in the 48 hours leading up to each masturbation session.
What happened? The researchers observed sharp increases in both ejaculate volume and the number of motile sperm from the men after they watched the seventh film. The time to ejaculation was significantly lower for the men as they watched the seventh film.
"It is possible that, by chance, the actresses or scenes randomly chosen for the novel trial for each participant tended to be ones that the men found more arousing," Sirot said. "We did not ask the men for a subjective rating of their level of arousal. However, our experiment was deliberately designed to try to eliminate any consistent differences between the video clips other than the novelty itself."
In case you're wondering what it took to get the guys to participate in a study like this, the answer is not much. As Sirot explained, "Each subject voluntarily participated for free and no compensation was needed."
Perhaps that's not such a surprise.
The study was published in the June 2015 issue of the journal Evolutionary Psychological Science.