I Asked 12 Scientists: What Is the One Fact Humanity Needs to Know? (2015)

The most import thing you'll ever know about humans: they are not actually rational.

Most of the time they act under emotional and / or subjective prerrogatives - not actual facts nor rational cost/benefits prerrogatives- commiting themselves to apparently rational actions within the rationality established by the emotional / subjective prerrogatives.

Sometimes, under certains circumstances - some predectible, some not - they act within rational constrains, but many of these are usually some cognitive bias in motion, not actual rational thought.

What humanity needs to know is the maximum number of humans that is indefinitely sustainable. At the US level of energy usage, that level is probably under one billion. At a European level of energy usage, we'd probably sustain about a billion and a half.

The current population level of just under 8 billion people is absolutely unsustainable, even at African Nomad levels of energy usage.