The findings aren't definitive and don't prove cause-and-effect, since it's possible that stressed-out men share another trait that disrupts their reproductive systems. Also, it's not clear if men with more stress are actually less fertile.
Still, "men who reported a higher number of stressful events happening in their life, or those who reported feelings of stress, had a lower percentage of sperm that moved well or were formed correctly, as well as lower numbers of sperm," said study lead author Teresa Janevic, an assistant professor at Rutgers School of Public Health's Department of Epidemiology in Piscataway, N.J. "Lower values of each of these measures are associated with lower fertility because of their reduced ability to fertilize an egg."
Prior research has linked stress to lower quality of semen, the fluid that contains sperm. But, "the results are not consistent across studies, and this is likely due to different populations and varied methods of measuring stress in these studies," said Audra Gollenberg. She is an assistant professor of public health at Shenandoah University College of Arts and Sciences in Winchester, Va., and was not involved in the new research.
The study was designed to "really get to the bottom of the question how stress is influencing the reproductive health of average men," Janevic said.
The researchers tracked 193 healthy men aged 38 to 49 from northern California who agreed to take part in a study and provide semen samples at a clinic. The men also answered questions about their levels of overall life stress and their stress at work.
The design of the study doesn't allow the researchers to link specific levels of stress to specific effects on sperm, Janevic said. And she added that "small increases in stress may not to be important for the semen quality in any individual man."
However, the study did find that men who were stressed in general had more dysfunctional sperm. This held true even after the researchers adjusted their statistics so they wouldn't be thrown off by traits like age, education level or race.
The researchers did not find a connection between work stress, specifically, and the quality of a man's sperm. "It's very possible that the levels of work stress in the men in our study were not high enough to see an effect," Janevic said. She did note that sperm were in worse shape in unemployed men compared to men who had jobs.
Gollenberg, the Shenandoah University professor, said other research has been inconsistent about the question of whether work stress might affect sperm quality.
"Work stress may be a different beast than life events, which are thought to be more traumatic and long-lasting," she said. "Some individuals are greatly affected by work stress, whereas others are able to leave 'work at work.'"
What might be going on biologically? It's not clear how a man's stress might affect his sperm, although scientists suspect that it has something to do with brain chemicals and testosterone. One theory suggests that stress in pregnant mothers could translate to sperm problems in their sons when they encounter stress, decades later.
So, what can stressed-out men do to ensure highly fertile sperm? The new findings don't say.
"We didn't study whether stress reduction can increase chances of conception," Janevic said. "However, stress reduction can contribute to health in various ways, and men can't go wrong to limit stress when possible as part of an overall healthy lifestyle."
Gollenberg pointed out that stress now won't necessarily translate to more sperm troubles now, since it takes about 2.5 months for sperm to develop. "Stress right now will affect semen quality in the next several months," she said.
The study was published online May 29 in the journal Fertility and Sterility.
There's more on male infertility at the American Urological Association.