Using a selection bias correction model with ordered probit, we estimate how a second semester of calculus affects students' grades in intermediate economic theory. Selection bias correction is needed because similar aptitudes and interests often lead students to enroll and do well in both mathematics and economics. A sample of students enrolled in 49 classes of intermediate micro and 41 classes of intermediate macro is used to estimate the model. The results show a predicted payoff from a second semester of calculus of about one whole letter grade in intermediate micro, but no payoff in intermediate macro.