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if I have just $10 to spend, and I'm presented with two students, one of whom is working four grades BELOW cohort and cannot seem to learn to read.... and the other of whom is working four grades ABOVE cohort and is ready for advanced mathematics/etc... my impulse is to give that funding where it is an investment for society, not just a momentary way of making myself feel awesome and charitable.

But if you're illiterate and/or don't graduate from HS, you're much more likely to struggle in myriad ways and to require costly support from the state, including AFDC, etc and, unfortunately, possibly jail time, which is very, VERY expensive. Investing in helping kids who are struggling in school actually makes tons of economic sense in ways that are probably a lot more easily quantifiable than investing in the gifted.