The Science of Giving

It was about nine on a weeknight.
"You got a dollar, man? Just trying to get home."
"Sorry, no cash."
"Oh come on man, just a dollar."
"No, I'm serious, I would give you a dollar, I just don't have one."
He didn't seem to hear me.
"I really don't have any cash. I give all the time, it's a problem."
"What?"


When ...