Changing major in the graduate school I am a second year under-graduate student studying mathematics, but also taking courses in economics. I am not sure at this stage if I will continue with maths in my graduate school. My option B is economics. Roughly, I would give 60-40 in favour of math as far as my graduate study is concerned. My question is this:
Which major should I focus for doing internship and research while I am doing my under-graduate  before applying to grad school?
Today your interests lie in economics, but what if by the time you finish, you're interested in computational biology? Good luck selling Economics → Computational Biology to a graduate school admissions committee. On the other hand, with Mathematics as your base degree, you can cross over to most disciplines including Economics (with your chances boosted by further by internships in that discipline). 

The bottom line is: the narrower your major discipline, the fewer your opportunities.