The author investigates how ability and gender affect grades on homework projects performed by assigned pairs of students in an undergraduate macroeconomics course. The assignment grade is found to depend on the ability of both students, and the relative importance of the stronger and weaker student differs in predictable ways depending on the kind of assignment. Male-male pairs earn lower grades than male-female or female-female pairs, controlling for the measured ability of the students. (Contains 6 tables and 10 notes.)