Drinking, smoking, being overweight and taking drugs have little impact on a key aspect of the sperm quality of infertile men, according to new research.
Men with unhealthy lifestyles produced as much swimming sperm as those living more sensibly.
The surprise findings contradict advice given to men who struggle to conceive.
Under current guidelines, GPs are supposed to warn men diagnosed with infertility of the dangers of alcohol, tobacco and recreational drugs.
Infertile men are also urged to avoid being overweight and not to wear tight underwear.
In some cases, fertility treatment is delayed to allow couples time to improve their lifestyles.
But the new research suggests none of these factors has much impact on the number of swimming sperm a man produces.
Scientists recruited 2,249 men from 14 fertility clinics around the UK and asked them to fill out detailed lifestyle questionnaires.
Information was then compared from 939 men who produced low numbers of swimming sperm and 1,310 men who produced higher numbers.
The results, reported in the journal Human Reproduction, showed that men with poor quality sperm were 2.5 times more likely to have had testicular surgery, and twice as likely to be of black ethnicity.