Talking about ourselves--whether in a personal conversation or through social media sites like Facebook and Twitter--triggers the same sensation of pleasure in the brain as food or money, researchers reported Monday.

About 40% of everyday speech is devoted to telling others about what we feel or think. Now, through five brain imaging and behavioral experiments, Harvard University neuroscientists have uncovered the reason: It feels so rewarding, at the level of brain cells and synapses, that we can't help sharing our thoughts.

'Self-disclosure is extra rewarding,' said Harvard neuroscientist Diana Tamir, who conducted the experiments with Harvard colleague Jason Mitchell. Their findings were published in the Proceedings of the National Academy of Sciences. 'People were even willing to forgo money in order to talk about themselves,' Tamir said.

To assess people's inclination for what the researchers call 'self disclosure,' they conducted laboratory tests to see whether people placed an unusually high value on the opportunity to share their thoughts and feelings. They also monitored brain activity among some volunteers to see what parts of the brain were most excited when people talked about themselves as opposed to other people. The dozens of volunteers were mostly Americans who lived near the university.

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In several tests, they offered the volunteers money if they chose to answer questions about other people, such as President Barack Obama, rather than about themselves, paying out on a sliding scale of up to four cents. Questions involved casual matters such as whether someone enjoyed snowboarding or liked mushrooms on a pizza. Other queries involved personality traits, such as intelligence, curiosity or aggression.

Despite the financial incentive, people often preferred to talk about themselves and willingly gave up between 17% and 25% of their potential earnings, so they could reveal personal information. 'We joked that this was the penny for your thoughts study,' Tamir said.

In related tests, the scientists used a functional magnetic resonance imaging scanner, which tracks changes in blood flow between neurons associated with mental activity, to see what parts of the brain responded most strongly when people talked about their own beliefs and options, rather than speculating about other people.

Generally, acts of self disclosure were accompanied by spurts of heightened activity in brain regions belonging to the meso-limbic dopamine system, which is associated with the sense of reward and satisfaction from food, money or sex.

'It rings true to me,' said psychologist Jamie Pennebaker at the University of Texas at Austin, who studies how people handle secrets and self-disclosure, but was not involved in the project. 'We love it if other people listen to us. Why else would you tweet?'