In newsrooms across America, tape clerks and film librarians will have spent the last few days retrieving video highlights of past US presidential debates ahead of next week's showdown between Barack Obama and Mitt Romney, in much the same way a sports reporter might piece together a showreel ahead of a grand final.

The footage will be familiar. Ronald Reagan gently scolding Jimmy Carter with his exquisitely timed 'there you go again.' Gerald Ford's monumental gaffe: 'There is no Soviet domination of Eastern Europe.' Lloyd Bentsen's devastating put-down during the 1988 vice-presidential debate, after Dan Quayle had made the callow mistake of comparing himself to JFK: 'I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy.'

Then there are those unspoken gaffes. George HW Bush looking impatiently at his wristwatch while debating Bill Clinton and Ross Perot in 1992. Al Gore, who was so heavily made up that he looked like a character from The Mikado, repeatedly sighing and harrumphing at George W Bush in 2000. Richard Nixon's flop sweat in 1960.

Were those same film librarians to go through the debates in 2004 and 2008, however, their search for explosive footage would come up empty. The same would be true for most of these televised head-to-heads. In all the hours of presidential debates – the first was in 1960, and the format was revived, after a 16-year break, in 1976 – there have been surprisingly few memorable moments.

Though televised debates tend to be judged now as presidential boxing matches and receive the same big fight hype, the debates are rarely decided by 'knock-out blows.'

A case in point is that first ever televised clash between Richard Nixon and Jack Kennedy. True, there are those grainy shots of Nixon's glistening upper lip (the vice-president had been recovering from a gammy knee, which he injured again on the way into the Chicago television studio). But even the most dedicated archivist would struggle to find a crushing soundbite. Rather, Kennedy was deemed to have won because he appeared so preternaturally calm, sounded authoritative across a range of issues and thus managed to close the 'stature gap' with his more experienced rival. The impressionistic nature of his victory was underscored by that oft-quoted poll of people who had listened on radio to the debate, who ruled Nixon the victor.

The paradox is that the presidential debates that have failed to produce dramatic exchanges sometimes end up having a bigger impact on the race. Like Nixon-Kennedy, the debates in 2004 between George W Bush and John Kerry produced no exchange that lives in the memory. Yet that first debate on 30 September shifted the polls. Bush had gone into it with a sizeable bounce from the GOP convention, and an average lead of 5.9%. Yet, as William Galston reminds us: 'Kerry's performance substantially exceeded expectations. Within a week, Bush's lead had fallen by more than two thirds, to only 1.8 points, and it stayed in that zone for the remainder of the campaign.'

Mitt Romney will be hoping for a similar outcome. The bad news for his troubled campaign is that the debates only rarely have a transformative effect on the race and have proved surprisingly inconsequential. The most authoritative academic study on the subject comes from the political scientist James Stimson, who studied polling data between 1960 and 2000. 'There is no case where we can trace a substantial shift to the debates', Stimson concluded.

Even a mega-gaffe like Ford's did not have an election-altering effect. And although John Kerry managed to eat into Bush's lead as a result of the debates, the President still ended up being re-elected.

There are various reasons why the debates are not as consequential as the lavish media coverage heaped upon them would suggest, not least their timing. By this late stage in the race, with less than fifty days to go before election day, most voters have made up their minds. Debates tend to reinforce views of a candidate rather than transform them. They tend to have a more race-altering effect in the primaries (as Texan Governor Rick Perry can attest) precisely because they are more formative. They are often the first time voters will have seen the candidates perform at length.

Inevitably, the first debate in Denver is being framed as Mitt Romney's 'one last chance', but don't be surprised if it ends in anti-climax. Historically speaking, most presidential debates have tended to be non-events.