In an article in The Proceedings of the Royal Society, two researchers described how mice in an experiment tended to get greater enjoyment out of rewards that were more labor-intensive:
Link | Photo by Flickr user Steve Berger Photography used under Creative Commons license
Mice were trained to push levers to get either of two rewards. Press one lever, out comes a drop of sugar water. Press the other and they get a drop of different tasting sugar water.
Then things got interesting. For one of the treats, scientists gradually increased the amount of effort required for the payoff—from one lever-press to five, then 10, then 15. So by the end of the session, one type of sugar water cost 15 times more effort than the other.
The mice then retired to their home cage where both treats were freely available. And they showed a strong preference for whichever reward they’d worked harder to obtain. Based on how fast the mice sipped, they appeared to find the costlier sugar water more tasty.
Link | Photo by Flickr user Steve Berger Photography used under Creative Commons license
"Why would I work so hard for this sugar water when the other one is so buch easier to obtain, It must be because I like it more!"
"And they showed a strong preference for whichever reward they’d worked harder to obtain."
The wording of that sentence implies they probably counterbalanced the conditions so that one group had to work harder for one flavour of sugar water, and the opposite for the other group.
They preferred the flavour that was rare.