While I believe that empathy and, with it often, generosity are natural to a human being, I think the tip example is not a good enough illustration of it.

There are 2 elements here that explain the phenomenon more than amply without adding inborn generosity.
1) social conditioning
2) self-judgment

I have seen this in action with my mother, who is not an ungenerous person, but she's never been conditioned to think that not leaving a tip is rude, greedy, or a form of cheating the waiter/waitress out of their payment (i.e. something that later may make her feel bad about herself). Where she comes from waiters are not tipped, tipping is/was a foreign custom taken upon by snobs who wanted to immitate foreign ways and furthermore impress their class superiority over the person being tipped. When I explained to her that it was part of waiters wages, she thought it was kinda silly, but she considered it and decided she doesn't want to tip. The waitress, in her view, did not perform any extraordinary services, wasn't extraordinarily pleasant or attentive. Furthermore my mom, decided that that she is actually much poorer than the waitress and need that $4 more. (Don't be aghast *I* left a tip).