People often ask, "What is the single most important environmental/population problem facing the world today?" A flip answer would be, "The single mo… - Jared Diamond

Our sex is ultimately laid down by our genes.

In fact, one of the main lesson to be learned from the collapses of the Maya, Anasazi, Easter Islanders, and those other past societies (as well as from the recent collapse of the Soviet Union) is that a society's steep decline may begin only a decade or two after the society reaches its peak numbers, wealth, and power. [...] The reason is simple: maximum population, wealth, resource consumption, and waste production mean maximum environmental impact, approaching the limit where impact outstrips resources. On reflection, it's no surprise that declines of societies tend to follow swiftly on their peaks.

Remember that impact is the product of two factors: population multiplied times impact per person.