The study was led by researchers from Columbia University's Mailman School of Public Health and Rutgers School of Public Health.
"Men who feel stressed are more likely to have lower concentrations of sperm in their ejaculate, and the sperm they have are more likely to be misshapen or have impaired motility," said senior author Pam Factor-Litvak, associate professor of Epidemiology at the Mailman School of Public Health.
"These deficits could be associated with fertility problems," Factor-Litvak said.
The researchers studied 193 men, ages 38 to 49, enrolled in the Study of the Environment and Reproduction at the Kaiser Foundation Health Plan in Oakland, California, between 2005 and 2008.
The men completed tests to measure work and life stress on subjective scale (how they felt overall) and objective scale (life events behind the stress).
They also provided semen samples. Technicians at the University of California, Davis, used standard methods employed in fertility testing to assess the samples for semen concentration, and sperm appearance and motility.
Measured subjectively or objectively, life stress degraded semen quality, even after accounting for men's concerns about their fertility, their history of reproductive health problems, or their other health issues.
Workplace stress was not a factor, however the researchers said it may still affect reproductive health since men with job strain had diminished levels of testosterone.
Unemployed men had sperm of lower quality than employed men, regardless of how stressed they were.
It is not fully understood how stress affects semen quality, researchers said.
It may trigger the release of steroid hormones called glucocorticoids, which in turn could blunt levels of testosterone and sperm production. Another possibility is oxidative stress, which has been shown to affect semen quality and fertility, they said.
"Stress has long been identified as having an influence on health. Our research suggests that men's reproductive health may also be affected by their social environment," said Teresa Janevic, the study's first author and an assistant professor at the Rutgers School of Public Health.
The study was published in the journal Fertility and Sterility.