How cooperative you are may depend on how quick you are to respond to a proposition, a new study finds. In a computerized game that involves contributing money to a common pool, people who took longer to think over their options were more likely to be selfish.

The question of whether humans' automatic impulses are cooperative or selfish is "a basic question about human nature," says David Rand, a Harvard University behavioral scientist and lead author of the new study. Psychologists have long tried to understand why people cooperate and in which situations. For example, when people play games repeatedly, there's an incentive to play nice, because other people will catch on to whether you're cooperating. But, Rand says, no one had looked at whether cooperation is a quick, intuitive choice or something that comes from greater deliberation.

For the new study, the researchers used Amazon Mechanical Turk, a website where people can sign up to do small amounts of work for small amounts of money. It's used for tasks like tagging photographs and transcribing text, and social scientists use it to run experiments. "Particularly for this kind of study where you're trying to understand people's intuition, it's really valuable to have easy access to a pool of people who are not all college undergrads_the normal subject pool for psychology and econ experiments," Rand says.

In several experiments, people played a single round of a decision-making game used by psychologists and economists called the public goods game. Each person was placed in a group of four subjects and given 40 cents. Each subject was then asked how much he or she wanted to contribute to a common pool. The subject was told that whatever ended up in the pool would be doubled and then divided among the four players.

So if everyone contributed their full 40 cents, then every player would double his or her money. (Sweet!) However, the game can reward greediness over cooperation: If a player donates nothing while the other three chip in their full 40 cents, then the stingy player will receive 60 cents from the pool and end up with a dollar total. These scenarios were part of the instructions that the players read. Of course, if nobody contributed, all of the players end up with just the 40 cents they started with.

People who chose quickly contributed an average of about 27 cents, while people who decided slowly contributed an average of about 21 cents. In later experiments, the researchers instructed some people to decide within 10 seconds and others to wait at least 10 seconds, to give them time to think more carefully. The same thing happened; fast people contributed more than people who deliberated. The researchers even double checked the results with young people in the lab_the normal behavioral science setting_and found that while people in the lab gave less overall, deciding quickly still made them give more. The results of the researchers' 10 experiments are published online Wednesday in Nature.

The researchers conclude that people are more cooperative when they think quickly. Psychology research shows that the faster choice is the more intuitive choice. "If they stop and think about it, they realize, Oh, this is one of those situations where actually I can take advantage of the person and get away with it," Rand says. He thinks that teaching people to make decisions rationally could actually make them less likely to cooperate with others. For example, courses that train business executives to overcome their impulses might also make them greedier. "We should try and keep our eye out for situations where we feel an initial impulse to do good, but on further reflection, rationalize and say we don't have to," Rand says.

So is it really surprising that people's first reactions tend to be more cooperative? "I think whether or not it is surprising depends on your prior beliefs," says Daniel Kahneman, a Princeton University psychologist who won the Nobel Prize in economics in 2002. "If you believe that people are wild and selfish, and then there is an overlay of civilization on top of that impulse, then the result is surprising." Simon Gauml;chter, an experimental economist at the University of Nottingham in the United Kingdom, notes that reflection didn't lead to pure selfishness. "Even in the most reflective conditions, people contributed some amount to the group." He says this suggests that people have a lot of cooperation in them but that gut reaction even adds a bit more.

This is adapted from ScienceNOW, the online daily news service of the journal Science.