Jen pointed me to Level-Headed: Economics Experiment Finds Taste for Equality. In brief, people are willing to pay their own money to take from the rich and give it to the poor. The underlying Nature article mentions that:

Emotions towards top earners become increasingly negative as inequality increases, and those who express these emotions spend more to reduce above-average earners’ incomes and to increase below-average earners’ incomes. The results suggest that egalitarian motives affect income-altering behaviours, and may therefore be an important factor underlying the evolution of strong reciprocity and, hence, cooperation in humans.

However, I can see other explanations that don’t require the explanation of altruism:

• Utility arbitrage: Utility is nonlinear: taking $1 when you have $10 of daily income is worse than taking $10 when you have $100 of daily income. This is used as an argument for progressive taxation, which might be nonlinear in money, but could be linear in utility (taxes giving everyone the same amount of pain). Those who take from the rich and give from the poor might effectively be doing arbitrage: the amount of gratitude from the poor minus the anger from the rich minus the cost amounts to a positive profit for Robin Hood.
• Insurance against slavery: There is an incentive for a commoner to prevent a powerful figure from gathering excessive power because letting this go on could lock the commoners into an under-caste.
• Power asymmetry: The rich can become richer only by increasing the imbalance in the income distribution. But as they become richer, they actually become fewer. At some point, increasing their riches actually reduces their power, and they get “taken under” (in a revolution or a revolt). Since revolutions are costly, it’s adaptive to “equalize” without breaking things up.

As an aside, it’s interesting to notice James H. Fowler among the authors: he’s behind a chain of very interesting papers over the past few years.