[nordsieck]

> If they didn't increase the fitness of their adherents, they'd die out rather quickly.

Counterpoint:

The Shakers, a Christian sect that enforced complete celibacy (pretty much the definition of decreased fitness) was founded in 1747 and apparently continues to this day [1] (although from the looks of it, it probably won't last another 50 years).

That being said, I largely agree with your point. Religions that make up substantial portions of society are probably beneficial for the fitness of their members.

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1. https://en.wikipedia.org/wiki/Shakers#20th_century_to_the_pr...