ument.” He urged parents to leave crying babies alone; to never hold them to give pleasure or comfort; and to kiss them only occasionally, on the forehead. Mothers were important less for their affection than as conditioners of behavior.

With a series of ingenious but sometimes disturbingly cruel experiments on monkeys, Harlow broke with this cool behavioralism. His most famous experiment showed that baby rhesus monkeys, raised alone or with same-age peers, preferred a foodless but fuzzy terrycloth surrogate “mother” over a wire-mesh version that freely dispensed meals. He showed that these infants desperately wanted to bond, and that depriving them of physical, emotional, and social attachment could create a near-paralyzing dysfunction. In the 1950s this work provided critical evidence for the emerging theory of infant attachment: a theory that, with its emphasis on rich, warm parent-child bonds and happy early experiences, still dominates child-development theory (and parenting books) today.

In the years since Suomi took over Harlow’s Wisconsin lab as a 28-year-old wunderkind, he has both broadened and sharpened the inquiry Harlow started. New tools now let Suomi examine not just his monkeys’ temperaments but also the physiological and genetic underpinnings of their behavior. His lab’s naturalistic environment allows him to focus not just on mother-child interactions but also on the family and social environments that shape and respond to the monkeys’ behavior. “Life in a rhesus-monkey colony is very, very complicated,” Suomi says. The monkeys must learn to navigate a social system that is highly nuanced and hierarchical. “Those who can manage this, do well,” Suomi told me. “Those who don’t, don’t.”

Rhesus monkeys typically mature at about four or five years and live to about 20 in the wild. Their development parallels our own at a fairly neat 1-to-4 ratio: a 1-year-old monkey is much like a 4-year-old human being, a 4-year-old monkey is like a 16-year-old human being, and so on. A mother typically gives birth annually, starting at around age 4. Though the monkeys copulate all year, the females’ fertility seasons are only a couple of months long. Since they tend to occur together, a troop usually produces crops of babies that have same-age peers.

For the first month, the mother keeps the baby attached to her or within arm’s reach. At about two weeks, the baby starts to explore, at first within only a few feet of its mother. These forays grow in frequency, duration, and distance over the next six to seven months, but rarely do the babies pass out of the mother’s sight line or earshot. If the young monkey gets frightened, it scampers back to the mother. Often she’ll see trouble coming and pull the infant close.

When the monkey is about eight months old—a rhesus preschooler—its mother’s mating time arrives. An

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