Hillary Clinton and Donald Trump are about to face off in one of the most anticipated debates in American political history. Clinton generally got good reviews as a debater in her two runs for the presidency. Trump’s debate performances during the Republican primaries were … unconventional. I won’t try and predict who is going to “win” tonight’s debate. But if history is any guide (and it may not be in this crazy election year), Trump has more to gain from the first debate than Clinton, though we shouldn’t expect a major change in the polls.

Four years ago, my colleague Nate Silver looked at the polls surrounding the first general-election debates in past presidential campaigns. Specifically, he looked at the average of polls in the week just before and just after the first debate in elections since 1976. I’ve updated that analysis with surveys from 2012. (The table lumps together incumbent presidents running for re-election and non-incumbent nominees from the party that holds the White House.)

Eight out of 10 times, the non-incumbent party’s candidate — that’s Trump this year — gained in the polls after the first debate. That includes each of the last five times. There are various theories to explain this. Some people think, for instance, incumbent presidents do poorly in first debates because they’ve had four years to grow unaccustomed to being challenged so directly, or that the challenger benefits simply by being on an equal playing field with the sitting president. Those theories don’t apply this year. That said, there are other reasons to think Trump has more to gain. He is currently winning a lower percentage of self-identified Republicans than Clinton is getting Democrats, so perhaps he has more lower-hanging fruit than Clinton: More Republican voters may come home after seeing both Clinton and Trump in action.

Another pattern that jumps out from the table above: First debates haven’t moved the polls all that much. Only twice did the leader change: Ronald Reagan moved ahead of Jimmy Carter in 1980 and won the election easily, and George W. Bush overtook Al Gore in 2000 and won the Electoral College (but not the popular vote). The average change in the margin between the two major-party candidates has been about 2.6 percentage points. Of course, with Clinton up by only 2 to 3 percentage points nationally, even a modest-sized Trump bounce could give him the lead.

There are also a couple of factors at work this year which could allow the first debate to swing the polls more than usual. First, it could be that at a time when social media can cement a narrative within minutes, a good debate performance matters more than ever. The largest first-debate gain since 1976 occurred in 2012 when Mitt Romney picked up 4.4 percentage points and drew into a near tie with President Obama in national polls. Four years before that, in 2008, Obama expanded his lead over John McCain by nearly 3 points after the first debate.

Second, there are an unusually large number of voters who are either undecided or favoring a third-party candidate at the moment. And the gains major-party candidates have made after previous debates have tended to come from those groups, rather than poaching the other nominee’s supporters. Even in 1980, when Reagan was deemed to have crushed Carter in their lone debate, Carter actually gained support after their meeting. Reagan simply gained more.

That year, third-party candidate John Anderson was kept out of the debate, and we have an analogous situation this year: Neither Gary Johnson nor Jill Stein reached the 15 percent threshold to be included in tonight’s debate. So it wouldn’t be too surprising if both Clinton and Trump gained support in the polls after tonight’s meeting. This could be good news for Clinton, given that Johnson and Stein supporters tend to favor her when forced to choose between her and Trump.

Remember, though, that there haven’t been that many campaigns featuring a debate and robust polling; we’re looking at only 10 to draw these conclusions, and that’s not a huge sample. And, moreover, there’s no guarantee that the gains a candidate makes after the first debate will stick. In 2012, Romney made up a ton of ground on Obama after the first debate, but those gains proved fleeting. In fact, the polls before the first debate, not just after, were far closer to the final outcome in 2012.

Here’s one thing we do know, however: The clock is ticking. The first debate usually has a bigger impact than the other debates, whether because there is less time left for voters to decide or simply because voters have already been reintroduced to the candidates. Indeed, tonight’s debate may be the last chance for major movement in the polls — barring some external shock. If you look at the polls taken after the first debate and the final pre-election polls, the average difference is just 2.2 percentage points. The leader in the polls after the first debate was always the leader in the final polls.

INCUMBENT PARTY AVERAGE POLLING LEAD YEAR POST-FIRST DEBATE END OF CAMPAIGN ABSOLUTE DIFFERENCE 1976 -3.0 -1.3 1.7 1980 -1.4 -3.8 2.4 1984 +17.0 +17.7 0.7 1988 +5.2 +9.0 3.8 1992 -13.5 -8.2 5.3 1996 +15.1 +12.1 3.0 2000 -1.5 -2.1 0.6 2004 +2.4 +1.0 1.4 2008 -6.0 -7.4 1.4 2012 +0.1 +1.5 1.4 Average 2.2 Polls don’t move that much after the first debate Sources: National Council On Public Polling, HuffPost Pollster

That means unless there is an October surprise, next week’s poll numbers will probably be close to the final standings. The best chance for the trailing candidate after that point is for the polls to be wrong (a real possibility), not for the polls to move.

Dhrumil Mehta contributed research.