Harry Harlow's experiments with infant monkeys transformed psychology's understanding of love, attachment, and emotional development, and Deborah Blum's biography reveals the complicated man behind the science. Harlow separated newborn rhesus monkeys from their mothers and gave them surrogate mothers made of wire or cloth. The babies clung desperately to the cloth mothers even when the wire ones provided food, demonstrating that contact comfort matters more than nourishment. Later experiments induced depression in monkeys through social isolation, producing disturbing images that provoked animal welfare protests. Blum, a Pulitzer Prize-winning science journalist, situates Harlow's research within mid-twentieth-century debates about child-rearing. Behaviorists like John Watson argued that mothers should avoid cuddling their children, that affection would produce weak adults. Harlow's monkeys refuted this claim with heartbreaking clarity. Babies need love not as a luxury but as a developmental necessity. The biography also examines Harlow's personal life: his marriages, his alcoholism, his depression, and his sometimes cruel treatment of students and colleagues. He could be charming and brilliant, petty and vindictive. Blum neither excuses the suffering his experiments caused animals nor dismisses the knowledge they generated. She asks readers to hold both truths simultaneously. The book illuminates how scientific paradigms shift, how researchers' personalities shape their questions, and how discoveries that now seem obvious once required experimental proof against determined opposition.