You'd expect poor kids to be good at self control (they're used to not be fed every 15 minutes) while rich kids being spoiled and not caring for an extra marshmallow.
Someone who's hungry has less will power than someone who's well fed.
- a poor man and a rich man are going to work when they each see a dollar coin on a sidewalk. Which of them would bother to pick it up?
- a poor man and a rich man are both drunk after spending a night at the bar. Which one would take a $50 taxi home, and which one will take a bus for $5 (you have to wait 15 minutes for a bus).
- a poor man and a rich man are both hungry and walk by a restaurant for rich people. It would take an extra 15 minutes to walk to a crappy diner for poor people, but a lunch costs 5x cheaper there. Where would each man dine?