Falling sperm counts are a "serious public health warning", and the trend could be linked to diet, lifestyle and possibly even tight underwear, a major French study has revealed.
The study showed that sperm counts and quality have fallen sharply since the start of the 1990s.
Between 1989 and 2005, average sperm counts fell by a third in the study of 26,000 men, increasing their risk of infertility.
The amount of healthy sperm was also reduced, by a similar proportion, the Daily Mail reported on Wednesday.
The study is important because, with over 26,600 men involved, it is probably the largest studied sample in the world.
The findings also confirm research over the past 20 years that has shown sperm counts declining in many countries across the world.
The fact that the decline was progressive over the 17-year period indicates the problem is ongoing, the researchers said.