"In 2017, author Shanna Swan and her team of researchers completed a major study. They found that over the past four decades, sperm levels among men in Western countries have dropped by more than 50 percent. They came to this conclusion after examining 185 studies involving close to 45,000 healthy men. The result sent shockwaves around the globe-but the story didn't end there. It turns out our sexual development is changing in broader ways, for both men and women and even other species, and that the modern world is on pace to become an infertile one. How and why could this happen?