According to this insightful exploration of how humans relate to temperature, warmth is essential for biological survival as well as the advancement of civilization. In a narrative that combines hard science and accessibility for general readers, social psychology professor Ijzerman, one of the world’s leading experts on “social thermoregulation in humans,” begins by describing studies in which subjects exposed to heat felt more sociable and kind than those exposed to cold. Later, however, he warns that when other scientists have repeated similar studies, the majority were unable to confirm the original findings. As a result, the author treads carefully, emphasizing large-scale research and acknowledging that “we psychologists are better at research than we are at giving practical advice and furnishing simple remedies.” Generating their own heat allows warmblooded animals to be incredibly active, but the process requires huge quantities of fuel; on the other hand, some snakes can go a year without food. Infants of all species seek warmth, and this quest persists throughout life. Although humans are intensely social, most of us do not understand the links among physical temperature and concepts of trust, friendship, and love. Even though many people believe in the universality of the connection between warmth and affection, “human cultures diverge with respect to affection-is-warmth.” Readers may be surprised by Ijzerman’s claim that “modern human relationships are organized around body-temperature regulation,” but he marshals impressive evidence in such chapters as “People Are Penguins, Too” and “Rat Mamas Are Hot.” It turns out that an infant’s search for warmth plays an essential role in attachment behavior later, and adults proactively seek it out, if not from physical proximity then through a romantic partner or social network. As the author shows, conventional wisdom about humans and warmth is often wrong. For example, studies do not confirm that weather influences our moods or that depression peaks in winter. Explaining thermoregulation for a popular readership may seem a stretch, but the author succeeds admirably.