The University of Nottingham study examined whether the possibility of punishing free-riders leads to better group results.

VOA: special.2009.03.03

What do you think about free riders?

你对免费搭车者有什么看法？

怎么协调意见冲突 - SpeakingMax英语口语达人

But there are ways to prevent free riders.

但搭便车问题，也有办法避免。

耶鲁公开课 - 公正课程节选