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November 9th, 2017

What motivated Conservative MPs to back or oppose Brexit?


Estimated reading time: 5 minutes


November 9th, 2017

What motivated Conservative MPs to back or oppose Brexit?


Estimated reading time: 5 minutes

The Conservative Party’s divisions over the EU are well known. But what motivated MPs to back Leave or Remain? Luke Moore uses logistic regression analysis to consider three key motivations: seeking office, votes, or that particular policy. He explains why all three affected Conservative MPs’ decision making, but that policy- or office-seeking were more prominent.

The divisions amongst Conservative MPs over Europe has been one of the most significant issues in modern British politics. Indeed, part of the reason why David Cameron called the referendum was to manage backbench dissent on Europe. The referendum campaign exposed deep divisions within the party with a majority of MPs backing remain but with a significant minority (41%) backing leave (see table 1).

Sources: MPs’ personal websites, BBC News, and

But what motivated their positions? MPs’ motivations are considered in terms of the policy, office, and votes trichotomy (Müller and Strøm 1999). This approach suggests that their positions can be explained by three objectives: MPs backed one side because they believed in the cause (policy-seeking); they backed a campaign in the hope of gaining promotion (office-seeking); or they based their positon on their electoral interests (vote-seeking).

Logistic regression analysis was used in order to test for the potential importance of these three motivations. Below I overview the results.


If Conservative MPs were policy-seeking it would be expected that ideology would be highly important in predicting referendum positions. Besides Europe, the other main ideological divide in the Conservatives in recent years has been the social/moral policy dimension which separates social liberals from social conservatives. Social conservatives hold more restrictive positions on issues such as abortion and gay rights, whilst advocating tougher punishment for criminals. Social liberals, by contrast, take a more permissive stance on social issues.

It was expected that social conservatives would be more likely to back leave. Previous studies of MPs and also voting behaviour in the referendum suggest that social conservatism is associated with Euroscepticism. The reason for this correlation cannot be established with any certainty. One plausible explanation is that positions on Europe could be related to the GAL (green/alternative/libertarian) versus TAN (tradition/authoritarian/nationalist) dimension of political contestation (Hooghe et al., 2002). Parties or politicians with TAN orientations often combine a strong defence of the national community with support for traditional values. On this basis it could be that a correlation between social conservatism and Euroscepticism signifies the importance of nationalism and a desire to protect British sovereignty.

To measure social conservatism I developed a variable which took account of MPs’ positions on five issues which resulted in a variable ranging from 1 to 10 with higher values denoting greater social conservatism. Figure 1 shows the spread of opinion on social conservatism amongst Tory MPs.

Higher values denote greater social conservatism. Social conservatism scores for each MP based on their positions on five issues: abortion, gay marriage, death penalty, use of three parent embryos and greater use of imprisonment as opposed to community sentences. For each issue, an MP was given 2 for a socially conservative stance, 0 for a socially liberal stance or 1 where there was insufficient data.

The results suggest that there was indeed a strong and statistically significant relationship between social conservatism and Euroscepticism. Figure 2 below plots this relationship.


Whilst the referendum campaign was formally unwhipped, meaning that ministers were not automatically dismissed for endorsing leave, the leadership made it clear that it would rather MPs supported remain. This gave MPs with ministerial ambitions an incentive to support remain.

It was predicted that MPs’ referendum endorsements would differ according to office-status. Frontbenchers had the most incentive to back remain as most of them would have wanted to either keep their jobs or gain promotion. Experienced backbenchers (who first entered Parliament prior to the 2015 general election) had the least incentive to support remain.

This group consists of MPs who had been in the Commons long enough to have been offered a government job but had either been rejected by Cameron, either through sacking or being overlooked, or had chosen not pursue a ministerial career, therefore this group had a lower expectation of being offered a position by Cameron post-referendum. MPs first elected in 2015 were expected to be in the middle of these two groups. The 2015 intake were too junior to have been offered a frontbench position. This group included many who were seeking promotion but also some who would have been less inclined to pursue office.

These differences were reinforced by the fact that a leave victory was likely to cause a Prime Ministerial resignation. Frontbench MPs owed their positions to Cameron and a new leader could threaten their jobs. By contrast, the experienced backbenchers had been rejected by Cameron and stood to potentially gain from a new leader. Indeed, the referendum did result in a new leader with Theresa May dismissing long serving ministers from the Cameron premiership, such as George Osborne and Nicky Morgan, whilst  appointing MPs who had been out of office for a long time, such as David Davis and Liam Fox.

The results were in line with these expectations. Figure 3 below shows referendum support by office-status. The only group with a majority (61%) supporting leave was the experienced backbenchers. Over three-quarters (76%) of frontbenchers supported remain. The new intake of MPs was in the middle of these two groups but slightly closer to the frontbenchers with 62% of MPs supporting remain.


It was predicted that the desire for re-election would mean that there would be a correlation between constituency Euroscepticism and the likelihood of supporting leave and that this would have the greatest effect on MPs from marginal constituencies. In order to measure constituency Euroscepticism, data from Hanretty et al was used.

The results suggest that there was a correlation between constituency Euroscepticism and endorsing leave. An experienced backbencher representing a highly Eurosceptic seat had a 72% chance of backing leave compared to only 38% for a similar MP from a highly pro-EU constituency. There is, then, evidence to suggest that vote-seeking did affect MPs’ referendum positions.

However, there are two caveats to this finding. The first is that there is no evidence that MPs from marginal constituencies were more sensitive to constituency opinion. The second is that the effect of the constituency Euroscepticism variable is weaker than the effect of either office-status or social conservatism. This means that an MP’s ideology or office status was a better predictor of their stance than constituency opinion. For example a socially conservative MP representing a pro-EU constituency is predicted to be more likely to support leave.

There were some unique factors in the Brexit debate which may have suppressed importance of vote-seeking. Firstly, as the result was determined by referendum, rather than by Parliament, MPs may have reasoned that constituents would not hold them responsible. Secondly, only 21% of MPs represented seats in which more than 60% of voters were estimated to vote the same way in the referendum, meaning that for most MPs, whichever side they backed, they would be opposing at least 40% of their constituents.


Policy, office, and vote seeking intentions all seem to have influenced the positions of Conservative MPs in the Brexit referendum. However vote-seeking intentions were less important than either policy or office-seeking.  The conclusion that policy and office-seeking mattered more than vote-seeking suggests that the internal politics of Conservative Party had a greater effect on MPs’ referendum positions than the wider electorate.


Note: the above draws on the author’s article published in Parliamentary Affairs.

About the Author

Luke Moore is a D.Phil. candidate in politics in the Department of Politics and International Relations, University of Oxford.



All articles posted on this blog give the views of the author(s), and not the position of LSE British Politics and Policy, nor of the London School of Economics and Political Science. Featured image credit: Pixabay/Public Domain.

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Posted In: Brexit | British and Irish Politics and Policy | Featured | Party politics and elections


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