Watching too much television and exercising too little might be at least partly to blame for a possible decline in the sperm counts of American men, a Harvard School of Public Health study suggests.

In findings published Monday in the British Medical Journal, the researchers assessed sperm quality in 189 college students ages 18 to 22 and found that those who exercised vigorously and often — more than 14 hours per week — and who watched no television had the highest quality and quantity of sperm compared with those who exercised less and watched more than 20 hours a week of television.