In fact though, the children who you reward later on think that this activity has less value and they are less likely to do it when there's no reward present.

但事实上，拿到奖励的孩子会想,这个活动的价值变低了，如果没有奖励,他们就不太可能会再做。

耶鲁公开课 - 心理学导论课程节选

I think it becomes more difficult.

我想就变难了。

学习是生活的一部分 - SpeakingMax英语口语达人