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. A 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 the race before the debates. Erikson and Wlezien conclude that evidence of debate effects is “fragile.”

Why are presidential debates so often inconsequential? After all, many voters do pay attention. Debates routinely attract the largest audience of any televised campaign event. And voters do learn new information, according to several academic studies. But this new information is not likely to change many minds. The debates occur late in the campaign, long after the vast majority of voters have arrived at a decision. Moreover, the debates tend to attract viewers who have an abiding interest in politics and are mostly party loyalists. Instead of the debates affecting who they will vote for, their party loyalty affects who they believe won the debates. For example, in a CNN poll after one of the 2008 debates, 85 percent of Democrats thought that Obama had won, but only 16 percent of Republicans agreed.

The impact of debates is also limited because the candidates are fairly evenly matched. Each candidate will have read a thick stack of briefing papers and rehearsed extensively. They will stick to their message and won’t be easily rattled. One candidate’s argument will be immediately countered by the other’s. Perhaps one candidate may appear more comfortable than the other. Perhaps one may momentarily slip up while the other does not. But the differences in their respective performances will be small. Candidates sometimes try to lower expectations of their own debate performance by claiming that they are just humble, plainspoken folks while their opponents are the second coming of Cicero. But Erikson and Wlezien’s analysis shows that across the series of debates in any given election year, the candidates tend to fight to a draw—much as one would expect two equally matched candidates to do.

Consider the first Kennedy-Nixon debate, which is remembered as anything but a contest between equals. In Theodore White’s famous recounting of the election, Kennedy appeared ”calm and nerveless”while Nixon was ”haggardlooking to the point of sickness.” Two Gallup polls suggest that after the debate Kennedy moved from 1 point behind Nixon to 3 points ahead, although it is difficult to know whether that shift is statistically meaningful. Both Stimson and Erikson and Wlezien find that Kennedy’s margin after all of the debates was only slightly higher than his margin on the eve of the first debate. Moreover, any trend in Kennedy’s favor began before the debates were held. Clearly 1960 was a close election, and many factors, including the debates, may have contributed something to Kennedy’s narrow victory. But it is difficult to say that the debates were crucial.

Ford’s erroneous assertion about eastern Europe in the second debate of 1976 is considered one of the biggest debate gaffes of all time. On the night of the debate, however, none of the debate viewers interviewed in one poll named the gaffe when asked about the ”main things” each candidate had done well or poorly. Only for viewers interviewed the next day did this gaffe become more salient—evidence that the public needed the news media to point out that Ford had made a mistake.

More importantly, Ford’s gaffe did little to affect the main trend in the fall campaign, which was a declining lead for Carter. According to Gallup’s polling, Carter had a 15-point lead before the first debate but only a 5-point lead after the second one. As Erikson and Wlezien put it, “Carter’s downward slide during the fall campaign seems to belie that this debate gaffe did much lasting harm.”

In 1980, the only debate between Carter and Reagan occurred a week before the election. Commentators judged Reagan’s performance favorably: it was “calm and reassuring,” wrote the New York Times’s Hedrick Smith the next day. A plurality of voters (44 percent) judged Reagan to be the victor, while only 26 percent picked Carter. Leading up to the debate, Reagan had about a 2-point lead, based on an average of the polls. He had a 5-point lead in the polls in the field on the day of the debate or in the two days thereafter. The debate seemed to matter, but it mainly nudged Reagan even further toward victory.

The 1988 debate between Dukakis and George H. W. Bush featured this famous question from moderator Bernard King: ”Governor, if Kitty Dukakis were raped and murdered, would you favor an irrevocable death penalty for the killer?” Dukakis said, ”No, I don’t, Bernard,” and then, in classic politician fashion, changed the subject to something he apparently did want to talk about: his record on violent crime as governor and his views about the war on drugs. His response was judged inadequately emotional, given that the question referenced his own wife. The postmortem in U.S. News & World Report said, ”The governor couldn’t summon a hint of emotion in his response to a jarring hypothetical question about the death penalty for someone who had just raped and killed his wife.” But voters couldn’t summon a hint of emotion about this alleged gaffe. Gallup reports that the two 1988 debates had “little to no impact on voter preferences.” Stimson estimates that these debates might have added a point to Bush’s margin, which would have only widened his lead, not handed him the election.

In 1992, George Bush’s glances at his watch in the October 15 debate with Bill Clinton and Ross Perot have been characterized, in one account, as a ”display of impatience” that ”seemed to speak volumes.” Once again, that gaffe — and, in fact, all of the debates in 1992 — had only a small impact on Bush’s standing. According to Thomas Holbrook’s

detailed study, the second debate may have cost Bush only about 2 points. If anything, these debates mainly served to increase Perot’s standing at the expense of Clinton’s — although Perot’s rise could also be attributed to other factors, including his own thirty-minute campaign ads during this period.

This brings us to 2000, which is a clearer case of a small, but consequential, debate effect. Al Gore’s performance in the first debate—with its interruptions of George W. Bush and audible sighs— was widely lampooned and is also considered by some to be one of the “biggest blunders” in the history of presidential debates. After the debate, there was a swing of 2 or 3 points toward Bush, enough to give him a narrow lead. Erikson and Wlezien estimate that after all of the debates, Gore’s poll standing was about 2 points lower than it was before. Among the many factors that influenced the outcome of the 2000 election, the debates appear to have been one.

But, even in 2000, this focus on presidential debates obscures an important point: debates aren’t the only thing that voters are hearing and seeing in the weeks before the election. So even a careful comparison of polls before and after a debate assumes, perhaps incorrectly, that any change was due to the debate itself or to news coverage about the debate—and not to other events, television advertising, or the like.

Moreover, other events may outweigh any debate effect. The 1980 election provides one example. After the debate and before the election, all of the following took place: prominent aides to both Reagan and Carter were forced to resign; economic data was released showing rising inflation; there was continued news coverage of the congressional investigation of Carter’s brother, Billy; and, finally, Carter was again rebuffed by Iran in his attempts to negotiate the release of the American hostages who had been held for a year. The Carter campaign’s internal polling showed Carter slipping even more after the setback in Iran than he appeared to be after the debate. ”It was all related to the hostages and events overseas,” said Carter’s pollster, Patrick Caddell. Reagan’s larger-than expected victory appeared to confirm that there was a late trend in his favor. Whether these events definitively hurt Carter in the closing days of the campaign is as difficult to determine as whether the debate helped Reagan. But the broader point remains: presidential campaigns present voters with a steady stream of information that may overshadow the debates.

A month ago, Obama’s advisers declared that they expect Mitt Romney to get “a surge of positive media attention and a boost in the opinion polls after the first presidential debate.” That may or may not prove true. What history can tell us is that presidential debates, while part of how the game is played, are rarely what decide the game itself.