Harry Harlow was a researcher who, along with John Bowlby and Mary Ainsworth, conducted foundational studies on attachment. In the 1950s, he performed a series of experiments on rhesus monkeys to investigate the mother-infant bond. By presenting newborn monkeys with two surrogate mothers—one made of wire that provided milk and another made of soft cloth that did not—Harlow discovered that the monkeys overwhelmingly preferred the cloth mother. They sought the wire mother only for nourishment but spent the majority of their time clinging to the cloth mother for comfort. This led Harlow to conclude that feelings of comfort and security, rather than just nourishment, are the critical components for forming strong maternal-infant bonds, which are essential for healthy psychosocial development.