Healthy sperm isn’t always a given for men. Quantity, movement, and structure all contribute to sperm health, and according to a recent study published in the journal Evolutionary Psychological Science, these traits may change with new sex partners.   

For the study, from the College of Wooster in Ohio sought to uncover whether men’s ejaculate traits changed in response to familiar or new female stimuli. A total of 21 heterosexual men between the ages of 18 and 23 were recruited to watch seven sexually explicit videos in a private room every 48 to 75 hours for 15 days.

The first six videos depicted the same actress and actor, while the seventh film had a different actress but the same actor. Each video consisted of a three-minute clip from a longer 20-minute video and was played on repeat until the men ejaculated.  

The men were instructed to record what time they began watching the film, what time they ejaculated, and whether all of their ejaculate was deposited into a collection cup. The researchers also assessed time of ejaculate volume and the number of motile sperm from each participant. If some of the ejaculate didn’t make it into the cup, it was not included as part of the data.

The findings revealed that the time it took participants to ejaculate ranged between four and 21 minutes. When it came to the first six films, there was no habituation effect, which means repeatedly viewing the same woman did not increase or decrease the time to ejaculation. However, they ejaculated faster and with a higher quality when viewing the seventh film, which included a new woman.

The researchers suspect these men were more likely to produce higher quality sperm for a new female stimulus for two reasons. First, they believe that hypothetically, the men had already fertilized the egg(s) or had their sperm stored by women they mated with. Second, these findings are linked to the concept of sperm competition, which refers to the competition between the sperm of two different men to fertilize a single woman’s egg.