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Why Is Sex Fun_

An evolutionary biologist explains why human sexuality—concealed ovulation, recreational sex, female menopause, coparenting, and body signals—is bizarre by animal standards and how those oddities were shaped by natural selection acting on the conflicting reproductive interests of men and women.

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What it’s about

Why do humans have sex for fun, in private, on days when conception is impossible, and even after menopause? Why don't men breastfeed, though they physiologically could? Why do women uniquely shut down fertility decades before death? Jared Diamond takes these seemingly natural facts of human life and reveals them as evolutionary puzzles that cry out for explanation. Drawing on comparisons with our great-ape relatives and the 4,300 other mammal species, he shows that human sexuality is molded by the same forces that shape any species' biology—relative parental investment, foreclosed reproductive opportunities, and confidence in parenthood—but produces a uniquely human outcome rooted in the built-in conflict between the sexes. Witty, rigorous, and provocative, the book uses recreational sex, menopause, and even the human penis to argue that our weird sexuality was as essential to becoming human as our big brains and upright posture.

The through-line

Who it’s for
A curious reader who wants to understand why human sexual behavior and their own body and desires work the way they do.
The problem
Human sexual features—recreational sex, concealed ovulation, menopause, male non-lactation, body signals—seem too natural to question yet defy easy explanation. The reader feels puzzled, sometimes distressed, by their own sexual drives and by conflicts between partners over sex and parenting.
The plan
  1. Step outside the human-centric perspective and compare human sexuality with other mammals and apes.
  2. Understand the battle of the sexes through relative investment, foreclosed opportunity, and confidence in parenthood.
  3. Apply evolutionary reasoning, not just physiology, to puzzles like male lactation and menopause.
  4. Use the comparative method across species to test competing theories of concealed ovulation and body signals.
  5. Recognize how each 'weird' human trait advanced gene transmission under ancestral conditions.
The payoff
The reader gains a coherent scientific understanding of human sexuality's strangeness. · They can distance themselves from destructive instincts and deal with them more intelligently. · They appreciate coparenting conflicts and body signals without moral judgment.

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