Do presidential debates really matter? Remember all the famous moments in past debates that changed the outcome of those elections? Well, they didn't.

• Author: Sides, John
• Position: TEN MILES SQUARE

When a presidential race is as close as this year's, there is endless speculation about what might tip the outcome to Barack Obama or Mitt Romney. One of the most anticipated events is the debates scheduled for October, which are already being hyped as potential "game changers." A common presumption about presidential debates is that one candidate can guarantee victory with a well-timed riposte or send their campaign into an irrevocable tailspin with an ill-timed stumble. After all, every political observer can point to truly "important" debates or moments during debates: the first televised debate between Kennedy and Nixon; the moment when Gerald Ford said, "There is no Soviet domination of eastern Europe"; Michael Dukakis's answer to the question about whether he would support the death penalty if his wife were murdered; George H. W. Bush looking at his watch; Al Gore sighing.

That presidential debates can be "game changers" is a belief almost universally held by political pundits and strategists. Political scientists, however, aren't so sure. Indeed, scholars who have looked most carefully at the data have found that, when it comes to shifting enough votes to decide the outcome of the election, presidential debates have rarely, if ever, mattered.

The small or nonexistent movement in voters' preferences is evident when comparing the polls before and after each debate or during the debate season as a whole. Political lore often glosses over or even ignores the polling data. Even those who do pay attention to polls often fail to separate real changes from random blips due to sampling error. A more careful study by political scientist James Stimson finds little evidence of game changers in the presidential campaigns between 1960 and 2000. Stimson writes, "There is no case where we can trace a substantial shift to the debates." At best, debates provide a "nudge" in very close elections like 1960, 1980, or 2000. An even more comprehensive study, by political scientists Robert Erikson and Christopher Wlezien, which includes every publicly available poll from the presidential elections between 1952 and 2008, comes to a similar conclusion: excluding the 1976 election, which saw Carter's lead drop steadily throughout the fall, "the best prediction from the debates is the initial verdict before the debates." In other words, in the average election year, you can accurately predict where the race will stand after the debates by knowing the state of...