The Happiness Hypothesis: Putting Ancient Wisdom to the Test of Modern Science

by Jonathan Haidt

Harlow put the milk hypothesis to a direct test. He created two kinds of surrogate mother, each one a cylinder about the size of an adult female rhesus monkey, complete with a wooden head that had eyes and a mouth. One kind was made of wire mesh, the other was covered with a layer of foam and then a layer of soft terrycloth. Each of eight baby rhesus monkeys was raised alone in a cage with two surrogate mothers, one of each kind. For four of the monkeys, milk was delivered only from a tube coming through the chest of the wire mother. For the other four, the tube came through the chest of the cloth mother. If Freud and Watson were right that milk was the cause of attachment, the monkeys should attach to their milk givers. But that’s not what happened. All the monkeys spent nearly all their time clinging to, climbing on, and pushing themselves into the soft folds of the cloth mother. Harlow’s experiment7 is so elegant and so convincing that you don’t need to see statistics to understand the results. You just need to see the famous photo, now included in every introductory psychology book, in which a baby monkey clings to the cloth mother with its hind legs while stretching over to feed from the tube protruding from the wire mother.