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But Kanazawa and Li’s savanna theory of happiness offers a different explanation. The idea starts with the premise that the human brain evolved to meet the demands of our ancestral environment on the African savanna, where the population density was akin to what you’d find today in, say, rural Alaska (less than one person per square kilometer). Take a brain evolved for that environment, plop it into today’s Manhattan (population density: 27,685 people per square kilometer), and you can see how you’d get some evolutionary friction.

Similarly with friendship: “Our ancestors lived as hunter–gatherers in small bands of about 150 individuals,” Kanazawa and Li explain. “In such settings, having frequent contact with lifelong friends and allies was likely necessary for survival and reproduction for both sexes.” We remain social creatures today, a reflection of that early reliance on tight-knit social groups.

The typical human life has changed rapidly since then — back on the savanna we didn’t have cars or iPhones or processed food or “Celebrity Apprentice” — and it’s quite possible that our biology hasn’t been able to evolve fast enough to keep up. As such, there may be a “mismatch” between what our brains and bodies are designed for, and the world most of us live in now.

Read why smart people are better off with fewer friends