Student or Learner
Researchers at Harvard University in the United States and the Stockholm School of Economics in Sweden did the study, which appeared last month in the journal Science．They had about two hundred college students play a version of the game known as the Prisoner's Dilemma．The game is based on the tension between the interests of an individual and a group．The students played in groups of four．Each player could win points for the group, so they would all gain equally．But each player could also reward or punish each of the other three players, at a cost to the punisher．
Q1: What does "in groups of four" mean? Does it mean that there are many groups, each of which consists of four people?
Q2: What does "at a cost to the punisher" mean?
Yes, each group had four students in it. There were many four-student groups.
I can make things bad for you, but it hurts me too - my hurting you comes at a cost to me.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.