One factor which could prove decisive in the referendum result is voter motivation, new research suggests. The data shows that yes voters are more likely to take part on 18 September and their greater appetite to cast a vote could add 2% to the final yes vote.

The data from the Scottish Social Attitudes (SSA) survey implies that in a tight contest come September, a stronger desire for independence may just make the difference.

That 2% difference may seem slim but the SSA data from the Scottish Centre for Social Research has another figure which will be cause for concern for the pro-UK campaign Better Together: amongst undecided voters, those leaning towards no are apparently 19% less likely to cast a vote than those leaning towards yes.

If that trend is maintained to polling day, that positive bias towards a yes vote could add 7% to the final pro-independence vote. If some recent opinion polls showing a far tighter gap between yes and no are correct (ICM recently suggested it could be as tight as just 4% apart, once don't know are excluded), that could be enough to ensure yes would win.

Dr. Jan Eichhorn, a research fellow at the University of Edinburgh involved in a number of in-depth referendum polling projects, said:

While the overall turnout in the referendum is expected to be high, around 70%- 80%, there could still be a higher turnout amongst Yes than amongst No supporters. In a tight race this could be crucial.

This will not be a surprise for Better Together: complacency or ambivalence amongst no voters has been their worry for months, particularly since many no voters expect Yes Scotland to lose. Those who dislike something or want change are more likely to be pro-active than those who are satisfied or unconcerned.

Alistair Darling, Better Together's chairman, told the Guardian in September, when the no vote was then healthier than it is now, the pro-UK movement had "a fight on its hands."

He added:

The worst possible thing [would be] for people to become complacent and think 'it's alright, it isn't going to happen'. A nationalist who fervently believes in nationalism is going to be at the polling station at five to seven in the morning, and is guaranteed to turn out. I don't want people who share my view to think 'I don't want to bother, the result's in the bag'. It's not.

There is an important health warning about this data: while the SSA is the most authoritative of the political polls and nearly 1500 voters were quizzed, these figures are more than seven months old. They are derived by the Scottish Centre for Social Research from interviews conducted between June and October 2013.

But then pro-independence campaigners might find greater cause for hope here too: the bounce in a yes vote in the wake of the backlash over George Osborne's currency union veto in February suggests more wavering Scots voters were pushed towards yes than no.

In a ScotCen commentary on these findings, Eichhorn said that based on four recent opinion polls, similar turn-out figures emerge:

A similar finding is to be found in recent opinion polls. On average the four most recent of the polls have indicated that yes voters are six points more likely to say they are certain to vote than no voters. If six per cent fewer no voters were to cast a ballot this would increase the Yes side's share of the vote by around one and a half points – a small but still potentially decisive difference should the contest otherwise be very close.

There is an important "ah, but" to add here. The ScotCen study also confirms trends seen in many polls to suggest that undecided voters, particularly those who do not identify with a particular political party, are less likely to turn out.

It is a mistake to assume an undecided voter will decide to vote: their indecision could be indifference, not an open mind.

They found that just over a third of those with no interest in politics – a number put at 12% of all voters by ScotCen's study, say there's a greater than 50% chance they will vote. The lowest ratio amongst those with party identification is amongst Labour voters: 15% say they are less likely to vote, compared to just 5% of SNP voters.

Eichhorn notes:

Perhaps unsurprisingly those who say they are undecided how they will vote are less likely to say that there is over a 50% chance that they will eventually cast a ballot. Only just over three quarters (77%) put their likelihood of voting that high and that proportion falls to just 70% amongst those who still say they are undecided even after they have been 'squeezed'. Evidently some of those who say they are undecided are going to be difficult to get to the polls at all, though even so as many as half of them (50%) say they are certain to vote.

He finds other demographic variations:

• Men are slightly more likely to say there is a greater than 50% they will vote, by 85% to 78%

• Fewer than 70% of voters under 35 say there is a greater than 50% chance they will vote, compared to nearly 90% of those over 45.

• For those newly franchised teenaged voters aged 14 upwards (chosen from 14 last year because by the time 18 September comes up, they will be 16 and eligible to vote), the voting "likely to vote" versus "unlikely to vote" rate was 66%.

• Social class matters too: only 73% of people in "semi-routine/routine" work (equivalent to Des) say there is a good chance they will vote, against 90% of those in managerial and employer roles.

Responding to these figures, Better Together said:

This analysis shows that everyone who believes that we are stronger and better together as part of the UK has to campaign for it and they have to vote for it. This is the biggest decision that we will ever take as a nation. It is too important to leave to other people.

A Yes Scotland spokesman said: