A Yale scientist leading a Harvard study has concluded that people are more generous when they must act on instinct than when they have time to adjudicate. He is quick to add, however, that the instinctive generousity may not represent a person's real choice. Per the article:

Despite the temptation to make conclusions about human nature, the new findings don't necessarily mean that people are naturally good and cooperative, Rand pointed out. He compared the automatic instinct to be generous with an impulse to eat a box of donuts. "If you think about yourself in situations where you have a first impulse and then stop to think about it, it's not at all clear that the first impulse is the real you," he said. "You want to eat the doughnut and then you're like, 'Well no, the doughnut is not good for me.' It's not like the real you is the doughnut-eater."

The analogy may not be valid because the instinct to eat a doughnut is a self-indulging one where the instinct to be generous is not.  

http://news.discovery.com/human/human-instinct-generous-120919.html