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"But what I'd really like to see is a really strong argument about human behavior that wasn't grounded in superstition."
Better to ground it in something real, right? There's great work on the evolution of the moral faculty; Marc Hauser recently wrote a book describing current findings in the field of evolutionary psychology, called "Moral Minds: How Nature Designed Our Universal Sense of Right and Wrong". The rough argument is: humans are the product of evolutionary processes; humans are social and moral creatures; there's good evidence that our ability to socialize has helped the species survive; so socializing and a moral sense are quite likely evolutionary adaptations.
There's a growing body of scientific evidence that supports conclusions like these. Pretty darn cool, I think!