"Blokes who wear boxer shorts are more likely to become dads, study reveals," reports the Sun, following research that looked at the underwear preferences of men at fertility clinics.
Researchers in the US recruited 656 men who were attending fertility clinics with their partners.
The men were given a questionnaire about their clothing habits, and provided sperm samples for analysis.
The researchers wanted to test the idea that choice of underwear affects sperm production.
The theory is that tight underwear can increase the temperature of the scrotum, which can reduce sperm production.
Men who reported that they usually wore boxer shorts had a 25% higher sperm concentration than those who reported usually wearing other types of underwear.
Men who wore boxer shorts also had a 17% higher total sperm count and 14% lower levels of a hormone that may be triggered when sperm production is reduced.
One point to bear in mind is that researchers looked at a very specific sample of men attending fertility clinics.
Many of these men may have had problems with sperm quality or quantity, regardless of the type of underwear they wore.
And we don't know if the results can be applied to men generally.
Because of the limitations of the study, it can't be concluded that tight underwear causes infertility.
But if you're trying for a baby, it can't hurt to try switching to boxers.
Other ways men can increase their chances of conceiving include moderating their alcohol consumption, stopping smoking, exercising regularly, and having a healthy, balanced diet.
The study was carried out by researchers from the Harvard T.H. Chan School of Public Health, Brigham and Women's Hospital, and Massachusetts General Hospital in the US.
It was funded by grants from the US National Institutes of Health and published in the peer-reviewed journal Human Reproduction.
Many of the headlines implied a far more direct relationship between underwear choice and male fertility than the research showed.
Neither BBC News' claim that "Ditching tight pants 'improves sperm count'", or the Sun stating that "Blokes who wear boxer shorts are more likely to become dads", were supported by the study.
This was a cross-sectional study in which people took part in the research at a single point in time.
This type of study can't tell us whether one thing causes another, as it's not possible to know which of the things being measured came first.
A cohort study would allow us to look at trends over time, particularly for men who didn't always wear the same type of underwear.
In theory, it could be possible to carry out a trial in which men were randomised to wearing particular types of underwear.
But allocating a large number of men to wear a certain type of underwear for a long time to see if this affected their success in conceiving isn't likely to be either ethical or feasible.
Researchers recruited men aged 18 to 56 who were attending the fertility clinic at Massachusetts General Hospital from 2000 to 2017 with their partner. None of the men had had a vasectomy.
The original study the researchers carried out looked at environmental influences on fertility and has already been published.
This research looked at the same group of men, this time with the question of whether underwear preferences influenced sperm production.
In the first phase of the study, all the men provided a sperm sample and a blood sample on the same day.
In the later stages of the study (from 2005 onwards), the researchers also looked at sperm samples the men had provided as part of their fertility investigations (in which 26% of all couples were said to have problems conceiving because of male health issues).
Sperm samples were tested for:
Blood samples from a subgroup of the men were also tested for follicle stimulating hormone (FSH), which is produced by the pituitary gland in the brain. FSH stimulates sperm production.
Rising levels of testosterone associated with sperm production cause levels of FSH to fall in a "negative feedback" system.
So higher levels of FSH may indicate that sperm production isn't occurring as normal and the body is trying to stimulate the testes to produce more.
The researchers didn't directly measure body temperatures in the scrotum or testicles.
Instead, the men were given a questionnaire to complete by themselves, asking them what style of underwear they wore most frequently in the last 3 months.
They were able to choose "boxers", "jockeys", "bikinis", "briefs" or "other".
Initially, 973 potential participants were identified, but 271 had to be excluded as they didn't provide information on their usual type of underwear.
Others were excluded if it turned out they didn't produce sperm at all or they had a history of cancer.
The final study featured 656 men, who between them provided 1,186 semen samples for analysis.
Boxers were the type of underwear that most men (55%) reported as their usual choice.
On average, these men tended to be younger, slimmer and more likely to take hot baths or use hot tubs compared with men who preferred other types of underwear.
Compared with men who wore all other types of underwear, those who usually wore boxers had a:
The researchers noted that their findings were consistent with previous studies, but their study did have some limitations.
These included the results only applying to the specific population studied, relying on men self-reporting their style of underwear, and the fact that other unmeasured factors could be causing variation in sperm quality.
They advised that future research should aim to confirm the findings in other groups of men.
The best type of underwear for healthy sperm production has been debated for a while.
This study provides some additional support for the idea that looser-fitting underwear may be better for producing healthy sperm.
But there are important considerations to keep in mind, many of which the authors acknowledged.
The study doesn't prove that men who wear looser underwear are more likely to become fathers.
We don't know:
The researchers reported that male infertility was the cause of problems for a quarter of couples presenting in latter years of the study, but don't give details on this.
They certainly didn't break down these 26% of men into the type of underwear worn, which may have been useful as these were actual cases of male infertility.
For the full sample of men, differences in sperm quantity and quality between those who wore boxers and the other groups were rather small.
We don't know whether any of the variation would ultimately have an impact on fertility.
To understand this further, we'd need a longer-term cohort study to follow up what happened to people after leaving the fertility clinic.
This study also didn't monitor sperm samples or underwear preferences over time: it simply asked what men tended to wear on average over the past 3 months.
But this could vary, and answers like "boxers" and "jockeys" could mean different things to different men. We can't be sure how accurate these responses are.
It's also possible that men in a fertility clinic setting aren't representative of men in the general population.
If you and your partner haven't conceived after a year of trying, it's recommended that you see your GP for advice.