The researchers gave volunteers a small amount of money along with the choice between giving all or some of it to charity or keeping some back for themselves. They determined that giving really did make the volunteers feel good. What's more, the NIH has the images to prove it:
NIH researchers used MRI scanners to observe patterns in brain activity in the participants as they made their decisions. When choosing to give charitably, the imaging study "strongly supports the existence of a 'warm glow' at a biological level," observed Dr. [Jorge] Moll, the lead researcher.
I love stuff like this, especially when it supports what I already believe. See, people who are cynical of charitable givers claim that donors give not to make a difference in the world but just so they can feel good. Now there are lots of reasons why people give to charity, some sacrificial, some self-serving. But according to the NIH study, the feel-good element isn't necessarily the motivation; it's simply a result of giving.
The most giving people I know have a peace in their lives that is enviable. They have very little to give in a material sense, but they donate their time and their talents and their wisdom to various charities.
And their "warm glow" is contagious—whether or not a scientific study can confirm that.