Why Is Sex Fun?: The Evolution Of Human Sexuality -

He explores why humans have "unnecessarily large" primary and secondary sexual characteristics—such as the human penis or female breasts—theorizing they serve as evolutionary "advertisements" for health, fertility, or social signaling. Where to Find the Book

In his 1997 book, , Jared Diamond argues that human sexual behavior is uniquely "bizarre" compared to almost all other animals. He posits that our unusual sexual traits were as essential to our evolution as our large brains and upright posture. Key Evolutionary Theories Why Is Sex Fun?: The Evolution of Human Sexuality

Humans are among the few social species that insist on having sex in private. Diamond argues this likely evolved to maintain social peace by reducing jealousy and conflict within cooperative groups. He explores why humans have "unnecessarily large" primary

While most animals remain fertile until death, human females stop reproducing mid-life. Diamond supports the "Grandmother Hypothesis" : older women provide a greater reproductive advantage by sharing wisdom and helping their children's offspring survive than by risking further late-life pregnancies themselves. Key Evolutionary Theories Humans are among the few

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Unlike most animals that only mate during fertile periods, humans have sex at any time—including during pregnancy or after menopause. Diamond suggests this evolved to strengthen pair bonds . Because men cannot tell exactly when a woman is fertile (concealed ovulation), they are incentivized to stay with one partner long-term, ensuring "paternal investment" to help raise vulnerable, slow-developing human children.