X THEN X: Manipulation of Same-System Runoff Elections

Do runoff elections, using the same voting rule as the initial election but just on the winning candidates, increase or decrease the complexity of manipulation? Does allowing revoting in the runoff increase or decrease the complexity relative to just having a runoff without revoting? We show that, for both weighted and unweighted voting, every possible answer can be made to hold, even for election systems with simple winner problems: The complexity of manipulation, manipulation with runoffs, and manipulation with revoting runoffs are completely independent, in the abstract. On the other hand, for some important, well-known election systems we determine what holds for each of these cases. For no such systems do we find runoffs lowering complexity, and for some we find that runoffs raise complexity.