Men who wear boxer shorts may boast higher sperm count than those who prefer briefs or jockeys, according to a significant new study.

The US research, involving 656 men who were seeking fertility treatment, showed that those who opted for boxers also had higher sperm concentration, and more swimming sperm per ejaculation.

This finding remained the same even after adjusting for other self-reported lifestyle factors, such as smoking, body mass index, and taking hot baths or Jacuzzis.

Interestingly, the study did not find any differences in sperm quality between the two groups.

Lead study author Dr Lidia Mínguez-Alarcón said this was the first time researchers had investigated the effect of underwear on reproductive hormone levels, which may affect sperm production.

By analysing DNA samples, brief or jockey-wearers were found to have higher levels of the follicle-stimulating hormone, or FSH, which kicks into overdrive when the body is overcompensating for lack of sperm production.

Sperm count was in the normal range for both groups, but higher for boxer wearers. Photo: AAP

High levels of the hormone are commonly found in people with testicular damage caused by increased heat in the region – suggesting wearing tight underwear (or any tight clothing, such as skinny jeans) could affect sperm count. A hypothesis that would need to confirmed by further research, Dr Mínguez-Alarcón said.

The study authors stressed the findings did not prove the type of underwear causes the difference in semen quality and FSH levels, only that that there is an association between them.

Fertility specialist Associate Professor Peter Illingworth told The New Daily this was the most convincing data he had seen confirming the difference between boxers and briefs, but cautioned the overall effects were small.

“Sperm count was in the normal range for both groups. They were just slightly higher in the men who were wearing boxers,” he said.

“The consequence of this on actual fertility is not clear.”

Normal sperm count is 15 million per millilitre of semen or more. At the lower end, the brief-wearers in the study had a sperm count of 46 million per millilitre, while those who opted for boxers had 58 million per millilitre.

The men, who were aged between 18 and 56, had an average body mass index (BMI) of 26, and had not had vasectomies.

Associate Professor Illingworth, who is also the medical director of IVF Australia, explained that male fertility is affected by many factors, not only sperm count.

“When we analyse sperm in a laboratory we look at the concentration, the movement, the shapes and the degree of DNA damage. It is only the concentration that was different,” he said.

“For couples who are struggling to fall pregnant, wearing boxers may increase sperm count fractionally.”

The study was published in Human Reproduction.