(Within the vocational majors, though, engineering is the highest-SES category. Business and education are both significantly lower. I don't know which of those would be most lucrative on average but I suspect it'd be engineering.)

I think there are several trade-offs there: engineering looks like the highest expected value to us, because we (on LessWrong, mostly) had pre-university educations focused on math, science, and technology. People from lower SES... did not, so fewer of them will survive the weed-out courses taught in "we damn well hope you learned this in AP class" style. And then there's the acclimation to discipline and acclimation to obsessive work-habits (necessary for engineering school) that come from professional parentage... and so on. And then of course, many low-SES people probably want to go into teaching as a helping profession, but that's not a very quantitative explanation and I'm probably just making it up.

On the other hand, engineering colleges tend to have abnormally large quantities of international students and immigrants blatantly focused on careerism. So yeah.