In the middle of the last century, two young scientists conductedexperiments that should have changed the world— but did not.Harry F. Harlow was a professor of psychology at the Universityof Wisconsin who, in the 1940s, established one of the world’s firstlaboratories for studying primate behavior. One day in 1949, Harlowand two colleagues gathered eight rhesus monkeys for a two- weekexperiment on learning. The researchers devised a simple mechanicalpuzzle like the one pictured on the next page. Solving it requiredthree steps: pull out the vertical pin, undo the hook, and lift thehinged cover. Pretty easy for you and me, far more challenging for athirteen- pound lab monkey.

Harlow’s puzzle in the starting (left) and solved (right) positions.

The experimenters placed the puzzles in the monkeys’ cages toobserve how they reacted— and to prepare them for tests of theirproblem- solving prowess at the end of the two weeks. But almostimmediately, something strange happened. Unbidden by any outsideurging and unprompted by the experimenters, the monkeys beganplaying with the puzzles with focus, determination, and what lookedlike enjoyment. And in short order, they began figuring out how thecontraptions worked. By the time Harlow tested the monkeys ondays 13 and 14 of the experiment, the primates had become quiteadept. They solved the puzzles frequently and quickly; two- thirds ofthe time they cracked the code in less than sixty seconds.

Now, this was a bit odd. Nobody had taught the monkeys howto remove the pin, slide the hook, and open the cover. Nobody hadrewarded them with food, affection, or even quiet applause whenthey succeeded. And that ran counter to the accepted notions of howprimates— including the bigger- brained, less hairy primates knownas human beings— behaved.

Scientists then knew that two main drives powered behavior. Thefirst was the biological drive. Humans and other animals ate to satetheir hunger, drank to quench their thirst, and copulated to satisfytheir carnal urges. But that wasn’t happening here. “Solution did notlead to food, water, or sex gratification,” Harlow reported.1But the only other known drive also failed to explain the monkeys’peculiar behavior. If biological motivations came from within,this second drive came from without— the rewards and punishmentsthe environment delivered for behaving in certain ways. This wascertainly true for humans, who responded exquisitely to such externalforces. If you promised to raise our pay, we’d work harder. If youheld out the prospect of getting an A on the test, we’d study longer.If you threatened to dock us for showing up late or for incorrectlycompleting a form, we’d arrive on time and tick every box. But thatdidn’t account for the monkeys’ actions either. As Harlow wrote, andyou can almost hear him scratching his head, “The behavior obtainedin this investigation poses some interesting questions for motivationtheory, since significant learning was attained and efficient performancemaintained without resort to special or extrinsic incentives.”What else could it be?

To answer the question, Harlow offered a novel theory— whatamounted to a third drive: “The performance of the task,” he said,“provided intrinsic reward.” The monkeys solved the puzzles simplybecause they found it gratifying to solve puzzles. They enjoyed it.The joy of the task was its own reward.

If this notion was radical, what happened next only deepened theconfusion and controversy....