University of Colorado at Boulder May 21 2026 Scientists have reported it for decades: Overpopulation can impair reproduction. Crowded chickens lay fewer eggs. Crowded mice have smaller broods. In humans, several studies have associated increased population density with reduced fertility. External factors, such as resource scarcity and social influences, undoubtedly play a role. But researchers have long suspected that intrinsic, biological mechanisms may also be at play as an evolutionary tool