Following up on a suggestion by Fabio Rojas, I went to this page and selected the study that looked at college graduates from 1999-2000 and interviewed them in 2001. I just did a simple table of annual job income by college major.
not in grad school 25th percentile
Pulling up the average are computer science, engineering, business, health, math, and vocational-technical. Pulling down the average are life sciences (why is this so low?), social sciences, humanities, physical sciences (again, why so low?), education, and "other technical/professional."
If the ultimate question is whether more students should attend and graduate from college, then perhaps what should interest me is something like the income of the lowest 25th percentile within each major. That might give a better idea of what we might expect to see at the margin if more students graduated college in the various majors.
At the 25th percentile, the really bad majors are life sciences (again, what are these, and why do they do so poorly?), humanities, and social sciences. Perhaps the 25th percentile of these three groups is what defines the unskilled college graduate. Note how much better vocational-technical looks at the 25th percentile.
[UPDATE: I re-ran the numbers, taking out of the sample anyone who was enrolled either part-time or full-time in a post-graduate program. See the far right-hand column of the table. The unskilled grads are not such extreme outliers if you do this, but they still are not doing very well.]