Feeling generous? Think it over a little and then see how you feel.

While humankind has often judged itself as greedy and self-interested, a growing body of evidence suggests that people's default mode is to be giving and cooperative -- particularly in times of stress. In fact, some behavioral scientists argue that ancient humans would never have survived their harsh environment and proliferated so successfully had they lacked the urge to help others.

A study in the journal Nature offers fresh proof that we are inclined toward collaboration -- by showing that the more time people were given to contemplate a generous act, the less likely they were to actually follow through with it.

"In many organizations there's a lot of emphasis placed on being as rational or reflective as possible and submerging personal instincts," said lead author David Rand, who studies human evolutionary biology at Harvard University. "What our results suggest is that the side effect of that is that people are more selfish."

'That's a good outcome'

Psychologists and economists have used such ominously named experiments as "the prisoner's dilemma" and "the dictator's game" to examine people's willingness to give gifts to strangers. However, this is one of the first studies to examine the relationship between generosity and time.

Rand and his colleagues recruited almost 2,000 people from around the world and paid them 50 cents to participate in an online "public goods game." Players were divided into groups of four and asked to donate some or all of their payment to a group project. All such contributions would be doubled and split equally among the four group members.

"If everyone contributes everything, everyone doubles their money, and that's a good outcome for the group," Rand said. "But from the individual's perspective, every cent you put in gets doubled and then split four ways, so you're only getting 50 cents back for every dollar you put in. The game creates this tension between what's best for the group as a whole and what's best for the individual. Some might decide to contribute nothing and take a free ride off the contribution of others."

Some test groups were forced to decide quickly, while others were given more time and prompts to reflect on their actions. Participants who decided quickly -- in 10 seconds or less -- contributed, on average, 67 percent of their money. Slower decision makers contributed 53 percent. In other words, when people were allowed to thoroughly consider their actions -- and employ reason -- they became increasingly stingy, authors said.

If its findings are accurate, the study could have wide-ranging implications for fundraising groups, governments and corporations, Rand said.

Risks and rewards varied for participants

While some scientists wonder whether impulsive generosity is an innate human quality, perhaps hard-wired in our genes, Rand and his colleagues wrote that they do not believe this is the case. The test subjects' reactions were impulsive, but likely shaped by years of interacting with other people and experiencing the rewards and costs associated with cooperation.

The study has caveats. One of the problems: about 40 percent of the participants were from the United States, 40 percent were from India and the rest were from other countries. As a result, the magnitude of the risks and rewards wasn't the same for all subjects.

The researchers were also unable to account for cultural differences among international online players, critics said.

In a commentary that accompanied the report in Nature, economist Simon Gaechter of the University of Nottingham in England noted that some people seem to be free riders while others are willing to help. He wrote: "The authors have demonstrated that, on average, our intuition is to cooperate, but further studies are needed to understand the variation in this behavior between individuals."