People are more generous if they don't stop to think through decisions, US researchers have found.
On the other hand, taking longer to consider a decision leads to a more selfish response.
Harvard scientists recruited participants to play what they called a "public goods game".
Subjects were put into small groups and faced with a choice: Keep the money they are given, or contribute it into a common pool that grows and benefits the whole group.
The researchers sought to find out whether people's gut reactions were more likely to be selfish or selfless. They started out looking at how quickly participants made their choices, finding that those who chose faster were more likely to contribute to the common good.
They then forced people to choose quickly or to stop and think. Again they found choosing quickly tended to result in less selfish decisions.
Finally, the researchers tested their hypothesis by manipulating people's mind-sets.
They asked some people to think about the benefits of intuition before choosing how much to contribute. Others were asked to think about the virtues of careful reasoning. Again the researchers found that intuition promoted cooperation, and deliberation did the opposite.
Their findings have been published in the September 20 issue of Nature.
Although the findings may appear to suggest that cooperation is innate or hardwired, the researchers said the study highlights the role of experience. People who had better opinions of those around them in everyday life showed more cooperative impulses in these experiments, and previous experience with these kinds of studies eroded those impulses.
"In daily life, it's generally in your interest to be cooperative," David Rand, a post-doctoral fellow in Psychology said. "So we internalise cooperation as the right way to behave. Then when we come into unusual environments, where incentives like reputation and sanctions are removed, our first response is to keep behaving the way we do in normal life. When we think about it, however, we realise that this is one of those rare situations where we can be selfish and get away with it."
Rand said the findings highlight an interesting and counterintuitive truth - that careful thought and reflection have a dark side.
Joshua Greene, the John and Ruth Hazel associate professor of the social sciences in the Department of Psychology, said: "When it's 'me' v 'us', our intuitions seem to work well. That's what's going on here."
"But what happens when people have different moral intuitions, for example, about abortion or raising taxes? When intuitions clash - when it's the values of 'us' v 'them' - reasoning and reflection may be our best hope for reconciling our differences."
Martin Nowak, professor of mathematics and biology, and director of the Program for Evolutionary Dynamics, said the capacity for cooperation has evolved over millions of years.
"These psychological experiments examine the causes of cooperation on a shorter timescale, on the order of seconds. Both perspectives are essential as we face global problems which require cooperation on a massive scale. We need to understand where cooperation comes from historically and how best to make it happen here and now."