Global sperm counts are falling. This scientist believes she knows why

On a rainy evening in Copenhagen last year, a diminutive woman in jeans, ankle boots and a casual shirt waited offstage at the Koncerthuset, a vast venue renowned for its acoustics. She had been invited by Science & Cocktails, a Danish non-profit that pairs lectures with drinks chilled in dry ice. Many in the audience were decades her junior and the mood was more rock concert than lecture as a voice over the loudspeaker announced, “The one and only — Shanna. Fucking. Swan!”

Swan, who turned 87 last month, walked on to the thump of a techno track, whoops and applause. “Wow. I have to say” — she chuckled gamely — “I’ve never had an introduction like that. And it’s wonderful.” As the hall quietened, she began to speak, calmly and without notes, about the animating purpose of her professional life. “I’m going to tell you a mystery story,” she said. “And hopefully, you’ll help me to solve it along the way.”

The mystery is this. Since the late 1930s, sperm counts around the world appear to have dropped significantly. While the decline was initially observed in western countries, there is evidence of the same phenomenon in the developing world, and it seems to be accelerating. Swan, a Berkeley-trained statistician-turned-epidemiologist, believes she knows why.

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