Shoppers on Edinburgh's famous Victoria Street. Shutterstock/Ssisabal

When the Labour government established the Scottish parliament in 1999, it hoped the new institution would demonstrate that Scotland’s distinctive needs and aspirations could be addressed within the framework of the UK. Consequently, the theory went, support for independence would melt away.

However, the project was not without its risks. As a symbol of Scotland’s distinctiveness, the parliament might foster people’s sense of Scottish identity rather than the British identity that helps bind the four parts of the UK together.

And by creating a space in which policy issues are discussed separately from Westminster – and sometimes different solutions implemented – the balance of public opinion north of the border might diverge from that in the rest of the UK, making it more difficult to keep the union together.

Ever since the parliament first met on July 1 1999, the Scottish Social Attitudes (SSA) survey has been charting the evolution of public opinion north of the border. Together with the British Social Attitudes (BSA) survey, it also provides a means of comparing the trend of public opinion on the two sides of the border.

A report published on October 9 by the Scottish Centre for Social Research and based on the 25 years of data collected by SSA shows that public opinion and identities in Scotland have not diverged from those elsewhere in the UK. Nevertheless, support for independence is higher now than 25 years ago. This is because some of the features of the country’s attitudinal landscape that were already distinctive to Scotland have come to be more closely aligned with whether people are for or against independence.

Consider national identity, for example. Acknowledgement of a British identity has always tended to play second fiddle to feeling Scottish. In 1999, two-thirds of SSA respondents said they were either “Scottish, not British” or “more Scottish than British”. Just 22% indicated they were “equally Scottish and British”, while only 7% stated they were either “British, not Scottish” or “more British than Scottish”.

The figures are little different in the latest survey conducted last autumn. Nearly three in five (59%) said they were wholly or predominantly Scottish, 22% (again) stated they were equally Scottish and British, while only one in ten (10%) claimed to be wholly or predominantly British.

It is often claimed that public opinion in Scotland is more leftwing than in England. The social attitudes data confirm this – though the gap is small and has not widened.

Both the Scottish and British surveys have regularly asked a suite of questions designed to ascertain people’s attitudes towards inequality and what the government should do about it. This produces a scale from 0 to 100, in which the higher the score, the more tolerant of inequality – and therefore more rightwing – somebody appears to be.

In 2000, the average score in Scotland was, at 34, just four points less than the equivalent figure for England (38). In our latest survey, the scores are 33 and 35 respectively.

Similar analysis of attitudes towards tax and spend shows that, while in any particular year, people in Scotland have usually been a little more likely to back more government spending and the taxes needed to fund it, the gap has not widened. Rather, attitudes have moved in parallel. When people in England have shifted away from tax and spend (or vice-versa), typically much the same shift has occurred north of the border.

Why the rise in support for independence?

Yet despite the absence of divergence in identity and policy preferences, support for Scottish independence is markedly higher now than when the Scottish Parliament was created. In 1999, just 27% said Scotland should become independent. As many as 59% backed having a devolved parliament, while just one in ten (10%) thought Scotland should not have any kind of parliament of its own.

Now, support for independence stands at 47%, while 41% back the devolved parliament and 9% do not want any kind of separate institution. Despite the 2014 vote against independence, the period before and after that ballot witnessed a sharp increase in support that has subsequently largely been sustained.

How has it been possible for independence to be more popular now even though the attitudes and identities of people in Scotland are no more distinctive now than in 1999? The answer lies in how some of the ways in which Scots’ attitudes and identities were already distinctive have become more closely aligned with their constitutional preferences.

People’s views on how Scotland should be governed have always reflected to some degree whether they feel Scottish or British. In 1999, only 6% of those who felt wholly or predominantly British said Scotland should become independent. In contrast, 44% of those who said they were “Scottish, not British” wanted Scotland to leave the UK.

Now, however, the link between people’s sense of national identity and their constitutional preference is much stronger. Support for independence among those who feel wholly or predominantly British is, at 14%, only eight points higher now than 25 years ago. In contrast, among those who say they are “Scottish, not British”, 74% now support independence, an increase of 30 points.

In 2000, those on the left on our scale (38%) were 15 points more likely than those on the right (23%) to say they supported independence. Now the gap is 34 points; 64% of those on the left are in favour, but only 30% of those on the right.

Public opinion and national identity in Scotland have not significantly diverged from the rest of the UK during the devolution years. Nevertheless, within Scotland, the constitutional debate has become more polarised.

No longer is it simply about how much sovereignty the country should have. Rather, it has become more strongly embedded in differences of identity and disagreements about the proper direction of public policy. That polarisation seems unlikely to make it any easier to find a lasting settlement to Scotland’s continuing constitutional debate any time soon.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: John Curtice, National Centre for Social Research

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John Curtice is currently in receipt of funding from the Economic and Social Research Council (ESRC) and has previously received funding from a range of government and charitable sources. The Scottish Social Attitudes survey is funded each year on a modular basis. This has included funding from both the Scottish and the UK governments, while the survey is currently in receipt of funding from the ESRC.