Senior author Pam Factor-Litvak, an associate professor of Epidemiology at the Mailman School of Public Health at Columbia University, and colleagues studied 193 men, ages 38 to 49, enrolled in the Study of the Environment and Reproduction at the Kaiser Foundation Health Plan in Oakland, Calif., from 2005 to 2008.
The men completed tests to measure work and life stress on a subjective scale -- how they felt overall -- and objective scale -- life events behind the stress. The study subjects also provided semen samples.
Stress, perceived stress and stressful life events, but not work-related stress, were associated with semen quality, the study said.
Workplace stress did not appear to be a factor, but the researchers said it might still affect reproduction because men with job strain had diminished levels of the male hormone testosterone. However, unemployed men had sperm of lower quality than employed men, no matter how stressed they were, Factor-Litvak 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," Factor-Litvak said in a statement.
"These deficits could be associated with fertility problems."
The researchers said they do not fully understand how stress affects semen quality.
The findings were published online in the journal Fertility and Sterility.