I think almost everyone has observed this. All you have to do is deliver a few pizzas to mansions and to trailer parks, and see where your tips come from, and you’ll see this in a action immediately.
There are several factors that may contribute here:
• Poor people know how far a few extra dollars go.
• Poor people tend to be less tightly wound, and generosity tends not to come from uptight people.
• Rich people got rich because they love money, and are thus prone to hoarding it.
• Lots of (though far from all) wealth comes from aggression and compromised integrity, which runs counter to kindness and generosity.
Could be other factors as well. Who knows?