I’m slightly puzzled by Murray’s analogy that “IQ is to success in life (or in college) as weight is to an offensive tackle in the NFL.” If Murray’s only point is that there is some IQ threshold below which it is nearly impossible to complete college, I agree. But the college threshold is vastly below the NFL threshold. Indeed, on p.152 of The Bell Curve, Murray himself estimates that a student of average IQ has about a 10% chance of getting a college degree.
In any case, if Murray wants to discourage marginal students from getting a BA, he doesn’t need to show that it is impossible for them to succeed. Threshold claims are a red herring. Even if the probability of completing college were linear in IQ, college could still have a negative expected return for many students. Why would Murray need to show anything more?
 For white students with average age and socio-economic status.