Men who feel stressed have fewer, slower sperm - which may make them infertile.
New research, published in the journal Fertility and Sterility, shows that psychological stress is harmful to sperm and semen quality, affecting its concentration, appearance, and ability to fertilise an egg.
Study senior author Dr Pam Factor-Litvak, associate professor of Epidemiology at the Mailman School of Public Health in the United States, said: "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.
"These deficits could be associated with fertility problems."
The researchers studied 193 men, ages 38 to 49. The men completed tests to measure work and life stress and found that life stress had a negative effect on sperm quality.
Dr Factor-Litvak said it is not fully understood how stress affects semen quality. 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.
The study's first author Dr Teresa Janevic, an assistant professor at the Rutgers School of Public Health, added: "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."