In the study, mice were trained to press levers to receive treats. Not only did the mice prefer the sugary treat that took 15 times more effort to get than the sugary treat received with just one press of the lever, but their preference remained the same when the researchers replaced the sugary treat with a low-calorie snack. When given free access to the same treats later, the mice who had to work harder for the low-calorie snack ate more of it and enjoyed it more than the mice who didn’t work nearly as hard. (Researchers measured their enjoyment by observing licking patterns.)
“It isn’t just about restocking expended energy, either,” Johnson says. “We tested them 24 hours after the training took place, and the preference held true even when there were no levers to press.” Although we’re not mice, the study does validate the long-held belief that we appreciate things more when we’ve worked for them.