Over the past century, the world’s human population has exploded from around 2 billion to 8 billion. Meanwhile, the average fertility rate has gradually declined. And if that trend continues as it has, we may soon see a crash in the population rate, which some argue could have disastrous effects.

Today on the show, we talk to co-authors Michael Geruso and Dean Spears about their forthcoming book After the Spike: Population, Progress, and the Case for People. Together, they explain why you should care about declining fertility rates.