Higher pressure air on the underside of a wing generally rolls outboard around the wingtip and over the top of the wing reducing lift and therefore increasing induced drag.
My intuition would tell me a wing with a dihedral would increase this effect as the higher pressure airflow on the bottom of the wing would flow outboard easier than anhedral. Is the opposite true with anhedral? Would the airflow flow inboard?
Are there any other factors that could contribute to efficiency?