Andrew G. Biggs, Abigail Haddad:

There are obvious advantages to going to college. And yes, science majors have much higher lifetime earnings than art majors. But the reasons why aren’t as simple as some studies would have you believe.
Whether to attend college and, if so, what to study are decisions of great financial and personal importance for younger Americans. It has become conventional wisdom that as many people as possible should graduate college and that college students should increasingly major in technical fields such as engineering, math and computer science. But college is a major investment. Average annual tuition at public four-year colleges today tops $13,000, with tuition at private schools exceeding $31,000. Moreover, the college major chosen by students guides the types of jobs they may hold for the rest of their lives, which influences not only income but also personal satisfaction from work. These choices should not be entered into lightly or lacking solid information.
Unfortunately, popular research on the costs and benefits of higher education is plagued by basic statistical errors, generating misleading conclusions and encouraging bad public policy. It is a basic tenet of statistics that correlation does not imply causation: simply because two things tend to occur together — such as college attendance and higher incomes — does not necessarily mean that one causes the other. While both college attendance and choice of major do affect earnings, their effects are much smaller than has been reported.