There are a lot of polite customs in this society that are not legally required. I am a very good tipper, especially at my regular places.
Yes!
I might have been six years old, and I noticed Daddy had left some change on the table (this would be 1956)
So, I collected it, and told him that he had left some money on the table. He then explained the concept to me, that the waitresses (waiters were unknown) made very low wage, and one always leaves some money for them.
A six year old got it.
Back to black folks being substandard tippers; it's just too bad that some academe thought this worthy of investigation. I question it all.
It is mostly likely not scientific - i.e. having a sufficient number of cases, and proper control on the experiment.
IOW, this guy/gal had an idea that needing proving. So it was.