A study was carried out by researchers from the University of Manchester, the University of Sheffield and the University of Alberta in Canada and looked at a very select group of men with fertility problems.
In the findings, researchers found no association between sperm motility and smoking, alcohol, recreational drug use or being overweight, although wearing tight underwear was associated with reduced sperm motility.
Around 40 in 100 men (recruited) had a low number of healthy sperm -- but these men were no more likely to smoke, drink, use drugs, or be overweight than men with normal sperm count. However, they were more likely to work in manual labor and less likely to wear boxer shorts.
Previous studies have suggested that wearing tighter underwear could slow sperm production by raising the temperature of the testicles.