Actually, it's not just blacks. It's the tendency in any groups of that size.

Formal analysis (game theory) indicates people tend to defect from a game (sort of cheat) when they feel safe and unidentifiable while feeling hidden in a group. The effect usually begins in groups of around 6 or 7 individuals, and is really pronounced once the group passes eight members.

So the †ack-on tip is intended to stop the defecting Timmy and Sally from not leaving a tip even though Billy and Terry and Ben and Stephanie did. (Although Billy and Terry only left a quarter and a dime and Stephanie and Ben only left pennies. But, who could really tell? And that's the point.

The effect occurs when people feel safe and hidden in a group. Reinhold Niebuhr also expanded on the effect in his book Moral Man and Immoral Society. In it, he explored the reasons people do things as groups that they never would as individuals.



Cheers!