American psychologist Harry Harlow studies the psychological development of monkeys. He gives a very young monkey two dummy monkeys: one made of wire but with a nipple from which it can suck, and one made of wood covered in skin. The baby monkey invariably attaches itself to the skin one, even when the wire one is heated with a resistor. Further studies show that monkeys and other mammals, including birds, develop mental disorders and maladjustment when separated from their mothers at an early age.
