Unless Monday's debate produces a dramatic shakeup it is tempting to think the race is trudging towards a predictable end

Both camps are going into Monday's debate hoping for a late break of opinion in their favour, but take the long view of support for Scottish independence and the impression is one of stability.

From 1999, when the Holyrood parliament first met, right up to this referendum year, the Scottish Social Attitudes survey records support for independence fluctuating around an average of about one third of voters. It has ranged between the peak of 38% (reached in 2005) and a trough of 24% (hit in both 2010 and 2012).

In 2014, however, the desire for Scotland to go it alone is right at the top of that range – 37%. And as the referendum campaign picked up over last winter, the momentum was very much with the yes side.

The chart below uses a "poll of polls": a rolling average of all the latest surveys, put together by psephologist John Curtice on the website What Scotland Thinks.

Last October, the yes/no split was 37% to 63%, suggesting that the traditional two-to-one balance against independence was holding in the referendum context. But then the nationalists began to narrow the gap. In February, Westminster's three unionist parties made a heavy-handed intervention on the question of the currency, warning that an independent Scotland could not count on keeping the pound.

This backfired, and by April, the poll of polls was running at 45%-55%. The race was looking increasingly competitive, before opinion congealed and then froze. Glasgow's Commonwealth Games and the first debate, which a Guardian/ICM poll established Alistair Darling had won for the no camp, came and went without materially affecting the picture.

And there is, Curtice says, consistency too in the detailed data. There is a persistent gender gap, with Scottish men readier than women to cut loose from England.

Older people, who retain a stronger sense of British nationality, are more unionist, and there is something of a class gap too.

"The more people have to lose materially the more cautious they are about making the leap to independence," Curtice explains.

Unless tonight turns out to have produced a dramatic shakeup, with just three weeks to go it is tempting to conclude that the race is trudging towards a predictable result. The SNP would, of course, dispute this – hoping for the same late surge that delivered its shock 2011 majority at Holyrood, and pointing to "wisdom of crowds" polling, which asks voters not about how they are planning to vote, but how they think the result will come out, and finds Scots expect a very close finish.

Against these nationalist hopes, however, the possibility of no-voter reticence must be weighed. Martin Boon is director of research at ICM, the pollster that pioneered an adjustment for "shy Tories" during the 1990s, which has improved general election predictions. Some voters who ultimately judged a Tory government would be best for them seemed to have found it socially undesirable to identify with an unfashionable Conservative party, and instead refused to reveal their intentions. With the energy and optimism of the yes campaign much admired, Boon suggests there is now at least a possibility of "shy noes".

But, as he explains, "whereas in general elections we can ask shy respondents what they did last time, and make some allowance for that, we've not had an independence referendum before, so we obviously can't do that here".

There is, then, no hard evidence on "shy noes". But if you buy the theory you would expect the race could be less close than some of the polls suggest.