Suppose you pay children in the 5th and 6th grades, right when you think of the achievement gap opening up between blacks and whites, to take an IQ test.
Say you have unmotivated black kids living in the middle of the ghetto and white kids from Scarsdale or some other upper-class neighborhood. You give each kid who gets a successful answer one M&M — just give them an M&M — and you say for each point extra on the IQ test, each correct answer, I’ll give you one more M&M. It turns out that the gap between the black and white student in the IQ test scores vanishes — vanishes completely.
If I’d heard this from almost anyone else, I’d be instantly skeptical. But I heard it from Jim Heckman, who sets the standard for caution and reliability in social science. I highly recommend the whole interview; you can read it here.