I have been searching for an article that I know exists, but somehow am unable to find.
The article is about an economic game where participants are assigned to a group, and can observe how much others have contributed to the common 'money pool.' According to the article, if people see the actual monetary amounts given by others, they will tend to like/not punish the person who has given the most. However, when they see the percentage of the incomes given to the money pool, and find out that the person who has given the most has actually given the least (percentagewise) from his income, they tend to severely punish him.
So, the authors conclude that people care about ratios given by others.
I do not know exactly which economic game this is, but I am certain of having read this.