From the The Boston Globe (June 15, 2008):

Surprising insights from the social sciences . . .

IF YOUR SON or daughter is not sure what to study in college, here's some research that just might help them in the long term, at least financially. An analysis of a large data set on college graduates confirms prior research (and common sense) that engineering degrees garner the highest income and arts/social science degrees the least, with business and science/math degrees somewhere in the middle. The analysis, however, also indicated when a second major can pay off. Two engineering majors or science/math majors are not worth more than one. Yet it always pays to add an engineering major to another kind of major. A business and science/math double major is worth more than either one by itself. It doesn't pay to add an arts/social science or education major to a business, science/math, or engineering major. And, finally, women get more out of engineering or science/math degrees, whether as single or double majors.

Del Rossi, A. and Hersch, J., "Double Your Major, Double Your Return?" Economics of Education Review (August 2008).