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Assumptions are part of our daily lives. And for good reason -- imagine if you had to qualitatively assess every aspect of your regular day. You assume that the food at the grocery store (and in your pantry) is edible. You assume that even the idiot in front of you will drive through a green light. You assume that you will, indeed, wake up tomorrow instead of dying in your sleep. These patterns lead to a certain degree of predictability. They happen to be right (most of the time), and therefore to spare some brain power for the surprises of the day, we assume them to be given.
Basically: it works this way most/all of the time. It makes sense this way. Why imagine it any other way?
When something deviates from our assumptions, it's abnormal. Weird. Strange. An aberration. Which is why we think so many animals have the weirdest mating habits. Mantises and some spiders eat the male during copulation? Chimpanzees' lady-butts swell during ovulation? We humans procreate just fine, thank you very much. Our system is tried and true. Why can't these other critters behave more normally, like us?
This is why I love to have assumptions challenged and perspectives placed in context. We assume humans are normal because we have to in order to function. But in so many ways, taken in the larger contexts of primate-kind, mammal-kind, and animal-kind, we are the freaks. The kooks. The evolutionary deviants.
From time to time, we all benefit from a little bit of reality check. Why Is Sex Fun? The Evolution of Human Sexuality puts our species into perspective and tries to riddle out why we're this particular kind of peculiar. The reframing of perspective is hijinx-level fun in your brain cavity.
(I have an issue with the book. It puts human culture -- that is, the evolution of our mental constructs -- on the same workbench as human and animal sexuality -- that is, the evolution of our reproductive constructs. Gauging how a "natural" human behaves is impossible, because the crunchy cultural coating we all wear gets in the way. Our cultural constructs are, on an evolutionary timescale, brand spankin' new. That is, our biology is practically identical to the way it was eleven thousand years ago. Yet our self-domestication is only ten thousand years old, as earmarked by the advent of agriculture.
Our culture has not yet had time to significantly affect our evolution -- and yet our variety of culturally-imposed sexual and reproductive habits are viewed in the book as evolved behavior. I don't see how Jared Diamond could have handled it any differently, because there are no "wild" humans. But I wish he had addressed the issue more thoroughly.
What I would like to see -- listen up, science nerds -- is an exploration of how our brain plasticity interacts with our sexuality. I think that approach could reconcile our reproductive and physiological evolution with our behavior.)