A very low IQ has a very clear and predictable effect on life. A very high IQ does not.

Ah, I see. Ever the optimist, I was imagining the low IQ folks had maybe found some unexpected ways to compensate.

Thanks.

Not OP, but I understood that to mean any difference in IQ below average (100) has a high impact on success, but differences above 100 have relatively less impact

I think their point is a low IQ nearly always means low success, but a high IQ doesn't always mean high success.