Reports of a decline in male fertility rely on flawed assumptions, a new study contends.

Rachel Gross:

For starters, no one knows what an “optimal” sperm count is. The World Health Organization sets a range of “normal” sperm count as from 15 to 250 million sperm per milliliter. (Men produce about 2 to 5 milliliters of semen per ejaculation.) But it isn’t clear that more is better. Above a certain threshold — 40 million per milliliter, according to the W.H.O. — a higher sperm count does not mean a man is more fertile.

“Doubling your sperm count from 25 to 50 million doesn’t double your chances,” said Allan Pacey, an andrologist at the University of Sheffield and the editor of Human Fertility. “Doubling it from 100 to 200 million doesn’t double your chances — in fact it flattens off, if anything. So this relationship between sperm count and fertility is weak.”