The famous experiments that psychologist Harry Harlow conducted in the 1950s on maternal deprivation in rhesus monkeys were landmarks not only in primatology, but in the evolving science of attachment and loss. Harlow himself repeatedly compared his experimental subjects to children and press reports universally treated his findings as major statements about love and development in human beings. These monkey love experiments had powerful implications for any and all separations of mothers and infants, including adoption, as well as childrearing in general. In his University of Wisconsin laboratory, Harlow probed the nature of love, aiming to illuminate its first causes and mechanisms in the relationships formed between infants and mothers. First, he showed that mother love was emotional rather than physiological, substantiating the adoption-friendly theory that continuity of care—“nurture”—was a far more determining factor in healthy psychological development than “nature.” Second, he showed that capacity for attachment was closely associated with critical periods in early life, after which it was difficult or impossible to compensate for the loss of initial emotional security. The critical period thesis confirmed the wisdom of placing infants with adoptive parents as shortly after birth as possible. Harlow’s work provided experimental evidence for prioritizing psychological over biological parenthood while underlining the developmental risks of adopting children beyond infancy. It normalized and pathologized adoption at the same time. How did Harlow go about constructing his science of love? He separated infant monkeys from their mothers a few hours after birth, then arranged for the young animals to be “raised” by two kinds of surrogate monkey mother machines, both equipped to dispense milk. One mother was made out of bare wire mesh. The other was a wire mother covered with soft terry cloth. Harlow’s first observation was that monkeys who had a choice of mothers spent far more time clinging to the terry cloth surrogates, even when their physical nourishment came from bottles mounted on the bare wire mothers. This suggested that infant love was no simple response to the satisfaction of physiological needs. Attachment was not primarily about hunger or thirst. It could not be reduced to nursing. Then Harlow modified his experiment and made a second important observation. When he separated the infants into two groups and gave them no choice between the two types of mothers, all the monkeys drank equal amounts and grew physically at the same rate. But the similarities ended there. Monkeys who had soft, tactile contact with their terry cloth mothers behaved quite differently than monkeys whose mothers were made out of cold, hard wire. Harlow hypothesized that members of the first group benefitted from a psychological resource—emotional attachment—unavailable to members of the second. By providing reassurance and security to infants, cuddling kept normal development on track. What exactly did Harlow see that convinced him emotional attachment made a decisive developmental difference? When the experimental subjects were frightened by strange, loud objects, such as teddy bears beating drums, monkeys raised by terry cloth surrogates made bodily contact with their mothers, rubbed against them, and eventually calmed down. Harlow theorized that they used their mothers as a “psychological base of operations,” allowing them to remain playful and inquisitive after the initial fright had subsided. In contrast, monkeys raised by wire mesh surrogates did not retreat to their mothers when scared. Instead, they threw themselves on the floor, clutched themselves, rocked back and forth, and screamed in terror. These activities closely resembled the behaviors of autistic and deprived children frequently observed in institutions as well as the pathological behavior of adults confined to mental institutions, Harlow noted. The awesome power of attachment and loss over mental health and illness could hardly have been performed more dramatically. In subsequent experiments, Harlow’s monkeys proved that “better late than never” was not a slogan applicable to attachment. When Harlow placed his subjects in total isolation for the first eights months of life, denying them contact with other infants or with either type of surrogate mother, they were permanently damaged. Harlow and his colleagues repeated these experiments, subjecting infant monkeys to varied periods of motherlessness. They concluded that the impact of early maternal deprivation could be reversed in monkeys only if it had lasted less than 90 days, and estimated that the equivalent for humans was six months. After these critical periods, no amount of exposure to mothers or peers could alter the monkeys’ abnormal behaviors and make up for the emotional damage that had already occurred. When emotional bonds were first established was the key to whether they could be established at all. For experimentalists like Harlow, only developmental theories verified under controlled laboratory conditions deserved to be called scientific. Harlow was no Freudian. He criticized psychoanalysis for speculating on the basis of faulty memories, assuming that adult disorders necessarily originated in childhood experiences, and interpreting too literally the significance of breast-feeding. Yet Harlow’s data confirmed the well known psychoanalytic emphasis on the mother-child relationship at the dawn of life, and his research reflected the repudiation of eugenics and the triumph of therapeutic approaches already well underway throughout the human sciences and clinical professions by midcentury. Along with child analysts and researchers, including Anna Freud and René Spitz, Harry Harlow’s experiments added scientific legitimacy to two powerful arguments: against institutional child care and in favor of psychological parenthood. Both suggested that the permanence associated with adoption was far superior to other arrangements when it came to safeguarding the future mental and emotional well-being of children in need of parents. [Estonian translation of this page by Boris Kozlow]