A recent study published in Evolutionary Psychological Science failed to find evidence that body mass index is be a strong predictor of sperm quality during intercourse in young, committed couples. The research examined potential links between the body size of both partners and various measures of ejaculate quality but found no significant associations.
Scientists have studied how physical traits influence reproduction across many species. In various animal models, males often produce more sperm when mating with larger females. This occurs because larger female size frequently signals higher fertility or a greater capacity to carry offspring.
Males in these species biologically adjust their investment to maximize the chances of fertilization. This strategy helps ensure that their genetic material competes successfully against potential rivals. Researchers have sought to determine if this evolutionary pattern holds true for human beings.
An influential study conducted in the early 1990s suggested that this mechanism might exist in humans. That research indicated that men inseminated more sperm when their female partners possessed a higher body weight. Despite the attention this finding received, the topic has seen limited follow-up investigation in the decades since.
“Research in nonhuman animals routinely finds that males inseminate more sperm into larger females, as this is considered a sign of greater fertility in females. This suggests that males strategically adjust their ejaculates to increase the chances of reproduction,” said study author Tara DeLecce, a postdoctoral researcher and special lecturer at Oakland University.
“The extent to which this applies in humans, however, is not well understood. In 1993, Robin Baker and Mark Bellis tested this relationship in humans and found that female weight was positively correlated with the number of sperm inseminated during copulation. No other study has investigated the relationship between female size and ejaculate quality in humans since then, so we were trying to replicate and expand on Baker and Bellis’ findings.”
“There is also a well-established body of literature on the detrimental effects of obesity on both male and female fertility, so we additionally wanted to better understand if weight is the best proxy for fertility in humans compared to other species.”
For their new study, the research team recruited 34 heterosexual couples to participate in the investigation. Most participants were attending a university in the Midwestern United States. The average age of the participants was approximately 21 years old.
To be eligible for the study, couples had to be in a committed relationship. The relationship must have been ongoing for at least three months prior to the study. Participants also needed to be between the ages of 18 and 35 to minimize the effects of age-related fertility decline.
The researchers utilized precise methods to measure body composition rather than relying on self-reports. They calculated the Body Mass Index for each individual by measuring height and weight in a laboratory setting. Researchers of the same sex as the participant performed these measurements to reduce potential anxiety.
Data collection involved analyzing biological samples produced during sexual intimacy. Couples were provided with specific non-latex, non-spermicidal condoms to use during intercourse. They were instructed to collect three separate samples over a period of 45 days.
The protocol required participants to abstain from ejaculation for at least 48 hours before collecting a sample. This abstinence period was capped at seven days to ensure standardization. Couples were also instructed to avoid scheduling collection sessions while the female partner was menstruating.
Participants followed strict protocols to maintain the integrity of the samples. They were required to deliver the samples to the laboratory within one hour of collection. The samples were kept close to the body during transport to maintain a temperature similar to internal body heat.
Participants wrapped the specimen containers in aluminum foil to assist with temperature retention. This procedure was intended to keep the sperm alive and active for analysis. Upon arrival at the laboratory, the volume of the ejaculate was measured immediately.
The samples were then analyzed using an automated system called the SQA-V. This machine uses electro-optical technology to assess various sperm parameters. It measured volume, concentration, and the percentage of sperm with normal shapes.
The system also assessed sperm motility, which refers to the ability of sperm to swim effectively. This is a key indicator of fertility potential. The researchers averaged the data from the three samples provided by each couple to create a reliable profile.
The analysis revealed that male Body Mass Index was not associated with ejaculate volume. There was also no significant link between male weight and sperm concentration. Men classified as overweight or obese did not differ significantly from normal-weight men on these metrics.
The study also found no relationship between male body size and sperm motility. The percentage of normally shaped sperm did not differ across the weight categories. These null results contrast with some prior studies that focused on men seeking infertility treatments.
The researchers then examined the potential influence of the female partner’s body size. The analysis showed no evidence that men produced higher quality ejaculates with partners who had a higher Body Mass Index. This finding does not support the “sperm competition” hypothesis derived from the 1990s research.
The data suggests that female body weight may not be a primary cue for adjusting sperm allocation in humans. It is possible that other traits are more relevant for signaling fertility in women. Traits such as waist-to-hip ratio might play a larger role than overall mass.
One statistical relationship did emerge from the data regarding the couples’ history. The length of the couple’s relationship was a positive predictor of ejaculate volume. Men in longer relationships tended to produce a greater volume of seminal fluid.
Relationship length did not predict other quality metrics like sperm morphology or concentration. The reasons for the link between relationship duration and volume remain open to interpretation. It may reflect unmeasured health or behavioral factors common in stable, long-term pairings.
The authors noted several limitations that provide context for these findings. The sample size of 34 couples was relatively small. A statistical power analysis suggested that hundreds of couples would be needed to detect small effects definitively.
The participants were also generally young and healthy university students. Previous studies that found links between obesity and low sperm quality often used older men or those with known health issues. The negative effects of obesity might be less visible in a young, healthy population.
Methodological differences between this study and the 1990s research might also explain the conflicting results regarding female size. The earlier study used manual counting methods rather than automated technology. The number of samples collected per couple also varied significantly between the two studies.
The findings imply that the relationship between body size and fertility markers is complex. Body Mass Index serves as a general proxy for body composition but may lack nuance. It does not distinguish between muscle mass and fat mass, for instance.
The study highlights the difficulty of replicating evolutionary psychology findings across different contexts. What holds true for insects or non-human mammals does not always translate directly to human physiology. Human mating behavior involves a distinct set of social and biological variables.
Future research is needed to settle the questions raised by this study. The researchers suggest that larger studies involving a more diverse range of body types are necessary. Such scale would provide a clearer picture of how body weight interacts with reproductive biology in the general population.
“According to our results, body mass index for both men and women is not significantly associated with the number of sperm men inseminate into their female partners during heterosexual intercourse,” DeLecce said. “However, this may be due to our small sample size that did not allow for the ability to detect statistically small effects.”
The study, “Male and Female BMI are not Associated with Copulatory Ejaculate Quality Among Committed Couples,” was authored by Tara DeLecce, Zachary W. Sundin, Bernhard Fink, and Todd K. Shackelford.
