What inspires humans to acts of generosity? Economists, psychologists and philosophers have pondered this question for millennia.

If one assumes that human behavior is primarily motivated by self-interest, it seems illogical to willingly sacrifice resources for others.

In an attempt to solve this paradox, some experts have theorized that giving satisfies a desire to boost one's standing in a group.

Others have suggested it fosters tribal cooperation and cohesion - a key element in mammal survival. Yet another explanation is that we give only because we expect to receive something in return.

The real answer, a study suggested Tuesday, may be much simpler: Giving makes us happy.

Scientists conducted an experiment with 50 people at a lab in Zurich who reported on their own happiness levels after acts of generosity.

Consistently, they indicated that giving was a feel-good experience.

At the same time, MRI scans revealed that an area of the brain linked to generosity triggered a response in another part related to happiness.

"Our study provides behavioral and neural evidence that supports the link between generosity and happiness," the team wrote in the journal Nature Communications.

Trial participants were promised an amount of 25 Swiss francs (23 euros or $26) per week for four weeks.

Half were asked to commit to spending the money on other people, while the rest could plan how they would spoil themselves. No money was actually received or spent by either group.

After committing to spending, the participants replied to questions while their brains were being scanned. The questions evoked scenarios pitting the participants' own interests against those of the beneficiaries of their experimental largesse.

The researchers examined activity in three areas of the brain -- one linked to altruism and social behavior, a second to happiness, and a third area involved in decision-making.

The group that committed to giving money away reported being happier than self-spenders, the team found -- even without having acted on their pledges.

The degree of happiness they reported was independent of the amounts they committed.

The findings have implications for education, politics, economics and public health, said the researchers.

"Generosity and happiness improve individual well-being and can facilitate societal success," they wrote.

"However in everyday life, people underestimate the link between generosity and happiness and therefore overlook the benefits of... spending" on others.

Fascinating questions remain.

"Can communication between these brain regions be trained and strengthened?" asked Soyoung Park of the University of Luebeck in Germany, who co-authored the study.

"Does the effect last when it is used deliberately, that is, if a person only behaves generously in order to feel happier?"

On Monday, a different study said humans were wired to help less fortunate people, but only if this does not upend the existing social pecking order.