When Bowlby was working on his theories about attachment, John B. Watson’s idea of “behaviorism” dominated psychology. Behaviorists believed that children learn through trial and error, so they would not necessarily respond if parents were overly affectionate or protective. In order to test whether children needed comfort and support, Harry Harlow separated infant monkeys from their mothers and gave them milk and warmth only. He found that these babies grew up to be emotionally disturbed and difficult to care for because they did not know how to behave normally. From this experiment, he concluded that human infants do not just instinctively seek comfort and safety, but also require nurturing and love.
In a series of experimental trials, Harlow found that infant macaques were able to develop attachment bonds with both types of surrogates—bare-wire and plush ones. However, the infants showed attachment-related behaviors only toward the plush mothers. This experiment confirmed and supported John Bowlby’s attachment theory.