There really is no such thing as a free ride — at least, not without reprisals, says a group of social scientists. James Fowler, an associate political-science professor at the University of California, San Diego, was intrigued by a paper published five years ago featuring a game that pitted people with different incomes against each other. In that game, each player had the option to spend his or her own money to make the whole group better off. Eventually, when each player had figured out the game's strategy, everyone became a 'free rider'. The rules were then altered, allowing people to punish free riders, who gained in wealth despite not contributing to the group. This caused the dynamics of the group to change. “Cooperation went up immediately, because people anticipated that they might be punished if they didn't contribute enough,” says Fowler.

However, Fowler and his colleagues were unsure of people's motives for punishment. Was it that people wanted to punish free riders or that moderate-to-low earners resented the richer players? So they designed a new game. “We asked what experiment we could do to see whether people are motivated to take down the top guy,” Fowler says. His team decided that the game should randomly assign incomes to each player and take away the option to contribute to the greater good, leaving only the options of punishing top earners and contributing to low earners — even though contributions would come out of each player's electronic 'pocket' and would not directly benefit them. “Overwhelmingly, people tended to bring down the top earner and bring up the bottom earner.”

The researchers studied sets of 20 people playing at computer terminals. Participants were split at random into different groups of four at various stages during the experiment. Players had access to information about the incomes of others in their group, but couldn't see each other or each other's computer screens.

Setting up the game was far from easy for Fowler and his co-workers. It was the most complicated game they had ever designed, because they had to manage and monitor interactions between multiple subjects. In addition, they didn't have time to test it properly before the experiment. “We decided to get volunteers into the lab quickly, then had to throw out some data because of bugs in the program,” Fowler says. Richard McElreath, who is based in the anthropology department at the University of California, Davis, where the game was played, tweaked the program, and the group was able to collect results from 100 people (see page 794).

Fowler suspects that the players' punishment of the rich stems from an evolutionary bent towards equality and cooperation. This has implications for society in terms of tax schemes or employment compensation. “You can encourage cooperation by making people more equal,” he says. He notes that increased income disparity in the United States during the past 15 years might actually have increased resentment towards the wealthy and decreased cooperation overall. A more progressive tax scheme might ameliorate that.

To weigh participants' emotional responses beyond the keystroke data, the researchers conducted exit interviews, asking: “Would it make you angry if you ran into people who earned a lot more than you?” The response was an overwhelming “Yes”. The players seemed to enjoy administering their own sense of social justice. “A lot of people asked if they could sign up again,” Fowler says. Many more people will soon have that opportunity; McElreath is making the game available from his website.