新托福考试阅读练习：MicePreferTreatsTheyWorkedHardertoGet

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　越是辛苦努力得到的成功人越觉其珍贵，也越懂珍惜。对于可爱的实验小鼠而言也是如此。  
" I' K% i4 L) A& r' `; T　　Maybe your dad said, “The harder you work for something, the more you’ll appreciate it.” Well, father really did know best. Because a new study finds that the harder mice have to work for a treat, the better it tastes. The research appears in the Proceedings of the Royal Society. [Alexander Johnson and Michela Gallagher, "Greater effort boosts the affective taste properties of food"]

8 b* ^8 y" \* D2 A, t3 _" K　　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.
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' i$ F1 S/ w" c. I# M3 r# p# A　　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.

　　Which makes evolutionary sense. An animal wouldn’t survive if a little hard work left a bitter taste in its mouth.