State of the art: party unity and parliamentary committees

The question of why MPs overwhelmingly vote together in parliamentary democracies has been subject to empirical studies especially for the last two or three decades. Theoretically, the literature identified a set of ‘pathways to party unity’ (Andeweg and Thomassen, Reference Andeweg and Thomassen2011), among them (1) homogeneity of preferences, (2) loyalty and (3) discipline. Voting unity is not determined by preference similarity among the MPs alone (Willumsen, Reference Willumsen2022), although MPs’ own ideological stances demonstrably affect their voting behaviour in certain policy areas (e.g., Degner and Leuffen, Reference Degner and Leuffen2016). The loyalty path to unity has been conceptualized as the result of MPs’ party socialization (Rehmert, Reference Rehmert2022; Mai and Wenzelburger, Reference Mai and Wenzelburger2023). To capture the varying effectiveness of party discipline, the career-related dependence of MPs on their party, compared to other principals (e.g., voters), has been approximated. Empirically, holding or ambitions to reach powerful offices (Kernecker, Reference Kernecker2017; Bhattacharya and Papageorgiou, Reference Bhattacharya and Papageorgiou2019; Zittel and Nyhuis, Reference Zittel and Nyhuis2019) strengthen, whereas an MP’s need for personal votes in the general election (Sieberer, Reference Sieberer2010; Sieberer and Ohmura, Reference Sieberer and Ohmura2021) or in intra-party primaries (Sozzi, Reference Sozzi2023), personalized campaign styles (Zittel and Nyhuis, Reference Zittel and Nyhuis2021), local ties (Tavits, Reference Tavits2009), economic interests of an MP’s constituency (Stiller, Reference Stiller2023), outside earnings (Mai, Reference Mai2022), a low prospect of being promoted to higher office (Benedetto and Hix, Reference Benedetto and Hix2007), career ambitions at another political level (Meserve et al., Reference Meserve, Pemstein and Bernhard2009), party switching (Gherghina and Chiru, Reference Gherghina and Chiru2014), impending retirement (Mai et al., Reference Mai, Link and Engler2023, but Willumsen and Goetz, Reference Willumsen and Goetz2017 for non-results) or being a ‘career politician’ (Heuwieser, Reference Heuwieser2018) weakens an MP’s propensity to toe the party line, arguably due to a changed effectiveness of party discipline compared to the baseline MP.

In contrast, empirical implications of division of labour as a fourth pathway to unity have rarely been included in observational studies of vote defections (Andeweg and Thomassen, Reference Andeweg and Thomassen2011; van Vonno Reference van Vonno2019). The division of labour principle in parliaments can be observed most clearly in MPs’ committee work. MPs’ issue specialization has effects on their re-election chances (Frech, Reference Frech2016), bill co-sponsorship patterns (Baller, Reference Baller2017), participation in parliamentary debates (Fernandes et al., Reference Fernandes, Goplerud and Won2019), communication with voters (Meyer and Wagner, Reference Meyer and Wagner2021), further career advancement (Cirone and van Coppenolle, Reference Cirone and van Coppenolle2018), attention towards particular issues (Borghetto et al., Reference Borghetto, Santana-Pereira and Freire2020) and their overall level of activity (Louwerse and Otjes, Reference Louwerse and Otjes2016). However, few studies focus on a connection between committee work and party unity. Based on interviews with German and Dutch MPs, Mickler (Reference Mickler2019) describes how the intra-party working groups develop their positions and why the whole party group usually follows them. Fujimura (Reference Fujimura2012) illustrates, for the Japanese case, that committee assignments are used to reconcile the party’s need for unity and MPs’ electoral concerns. For the U.S. case, Kanthak (Reference Kanthak2009) shows that MPs having received plum committee assignments toe the party line more frequently than other MPs. Additionally, Grimmer and Powell (Reference Grimmer and Powell2013) find that legislators who were involuntarily removed from preferred committees have significantly higher defection rates. Conversely, MPs overwhelmingly toeing the party line receive more attractive committee seats or even chair positions, compared to more rebellious colleagues (Leighton and López, Reference Leighton and López2002; Cann, Reference Cann2008; Whitaker, Reference Whitaker2019; but see Chiru, Reference Chiru2020 and Fernandes et al., Reference Fernandes, Saalfeld and Schwemmer2022 for non-results).

Although those findings are related to division of labour, not the MPs’ specialization itself but a changed effectiveness of party discipline after having gained or lost desired committee seats seems responsible for the variation of party unity. Committee assignments are one of the ‘carrots and sticks’ the party leadership uses to enforce discipline (Bailer, Reference Bailer2018). Only Willumsen and Öhberg’s (Reference Willumsen and Öhberg2017) study of the Swedish case explicitly links MPs’ issue specialization to their voting behaviour and finds that MPs vote less frequently against the party line concerning issues that have been dealt with in the parliamentary committee they belong to. We will complement their argument in three crucial respects: First, we argue that not only the current but also past committee memberships are likely to affect individual legislative behaviour. Second, we expect behavioural differences between policy spokespersons and ordinary committee members. Third, we posit that the effect of committee membership is conditional on the importance of the vote. In the subsequent section, we theorize on those aspects and derive five falsifiable hypotheses.

Theory

In its 19^th term (2017–2021), 870 draft bills and hundreds of other motions (e.g., amendments, resolutions) were introduced and (most of them) adopted or rejected during votes in the German Bundestag. The MPs were provided with more than 31,000 documents as a technical basis for their decisions. However, they are only able to take note of a small subset of those documents and to form their opinion on only some of the motions brought to the floor (Ismayr, Reference Ismayr2012). Most obviously, time constraints prevent them from doing so – as described by a leading MP cited in Searing (Reference Searing1995: 680): ‘The volume is so great that most of the time they would be completely lost, they wouldn’t know which way to vote’. In order to keep the amount of information to process manageable, MPs usually join one or two standing committees that, broadly, resemble the jurisdictions of the government departments and deal with bills and other motions related to them (Siefken, Reference Siefken, Siefken and Rommetvedt2021). Within each parliamentary party, the committee structure is reflected by working groups (Arbeitsgruppe) in which the MPs collaborate in defining their party’s position. In smaller parties, the working groups consist of MPs from multiple committees with similar topics, e.g., foreign, defence and development aid policy (Arbeitskreis; Mickler, Reference Mickler2019). The positions of the party’s working groups mark an important predecision for a motion’s prospect to be passed. Suppose MPs or party factions aim to modify a proposal: In that case, not the plenary, the committee or the party group meeting but the responsible working group of the parliamentary party is the suitable arena for such an undertaking (Schöne, Reference Schöne2010). If its members have agreed on a common position, a discussion in the whole party group is ‘preferably avoided’ (Mickler, Reference Mickler2019). Consequently, membership in those working groups is the most promising avenue for backbench MPs to pursue their policy-seeking goals.

There are several reasons why MPs do not often disagree with the other policy specialists of their party regarding their own topics. Besides other factors, committee assignments and, consequently, MPs’ ‘own’ topics often reflect their educational or occupational background (Mickler, Reference Mickler2018). In the German parliament, for example, members of the agricultural committee are to a disproportionately higher share farmers themselves; most of the members of the justice committee are lawyers (Ismayr, Reference Ismayr2012). Additionally, according to the distributive theory of legislative organization, MPs systematically join committees related to constituency characteristics: MPs from poor districts are often members of social policy committees, whereas members from constituencies with an increased demand for construction activity join infrastructure committees (Martin and Mickler, Reference Martin and Mickler2019 with further references). Both the personal and electoral reasons for their committee membership might favour a common understanding of problems and, possibly, even a rough homogeneity of preferences among the policy specialists of a party group. Within the cooperative structures of their working group, the single MPs are described as ‘busy, well-informed, often experienced, more or less knowledgeable policy workers who develop and evaluate parliamentary motions’ (von Oertzen, Reference von Oertzen2006: 256, own translation). Within their role, they are able to introduce their personal preferences into the decision-making process of their party group – in some cases, even proactively and not only related to substantiating, modifying or impeding government motions (Schöne, Reference Schöne2010). Consequently, because those policy specialists are involved in shaping their party’s position, they could reduce the policy distance between them and the responsible working group of the party and, finally, the whole party group. Since the MPs thus helped to shape the party line within their area of specialization, they have fewer reasons to vote against it (Willumsen and Öhberg, Reference Willumsen and Öhberg2017). Dingler and Ramstetter’s (Reference Dingler and Ramstetter2021) inverted finding whereby MPs have higher defection rates within their focus area might be rooted in that the authors measure MPs’ interest (by parliamentary questions) and not their capability of shaping policies.

The negative effect of committee membership on defection rates could be amplified by the fact that among the policy specialists who have often worked together for many years there is an increased expectation of mutual loyalty and not to distinguish oneself by dissent from the responsible working group (Schöne, Reference Schöne2010). After some Christian Democratic MPs had voted against measures to resolve the Eurozone crisis in 2015, the leader of the party group threatened them with their removal from the respective committees:

In the year before, another member of this party lost his rapporteur position after publicly proposing social policy reforms that were not coordinated with his working group (BILD, 2014). Both examples illustrate that the leadership expects MPs not to take deviating stances on topics which they work on in their respective committees.

Hence, most dissenting votes are likely to happen outside their areas of specialization. MPs not familiar with a policy field might lack the expertise to have their own view on a particular motion (van Vonno, Reference van Vonno2016). Besides that, MPs might also not have the same interest in every decision and may ignore certain topics (Schöne, Reference Schöne2010). If they do not have an opinion on an issue, MPs take voting cues from the policy specialists of their party. However, if MPs actually have an opinion and are not members of the respective committee and intra-party working group, they have a greater leeway to dissent. Additionally, there are more substantive reasons for defections, as these MPs were not involved in shaping the party line.

That said, we expect that not only the current committee membership but also which topics MPs have specialized on in the past influences their legislative behaviour. Former committee membership indicates interest in the policy area, and MPs keep at least parts of the expertise gained in the past. Therefore, if MPs have been committee members one or two legislative terms ago and, after that, turned to another policy area, they most likely still have an opinion on topics of their former committees. Additionally, during their former membership in a committee and the respective intra-party working group, a convergence might have taken place between the MPs’ preferences and the party line which those MPs used to shape in former times. Lastly, MPs might still feel a sense of loyalty towards their former working group and its members whom they know from their prior collaboration. To conclude, we expect the policy-shaping hypothesis to be valid not only for MPs’ current but also for their past committee memberships – which has not been empirically tested before.

As insiders of the parliamentary business (von Oertzen, Reference von Oertzen2006), the main tasks of policy spokespersons are to lead the working group concerning a particular policy area and to express the working group’s position vis-à-vis the committee, the media and, secondarily, the policy spokesperson of the coalition partner (Ismayr, Reference Ismayr2012; Mickler, Reference Mickler2019). One of their primary duties is to advocate the position of their working group within the whole parliamentary party (von Oertzen, Reference von Oertzen2006). In this respect, policy spokespersons have, compared to ordinary MPs, significantly better means to shape the party line. Acting as a first filter, they can push certain initiatives or delay others (von Oertzen, Reference von Oertzen2006). Additionally, since they are members of the party group leadership, they have an informational advantage and a certain leeway in settling conflicts with other intra-party working groups. Their informational advantage is also rooted in the fact that they have privileged access to the parliamentary staff of their party and are able to use its expertise for their purposes (Petersen and Kaina, Reference Petersen and Kaina2007). If the working group is divided on an issue, the policy spokesperson most likely has the authority to resolve conflicts with their decisive vote. This is reflected by the MPs’ own perceptions: 68% of the German MPs who took part in the survey support the statement that the spokespersons define the position of the (whole) party group (van Vonno, Reference van Vonno2016). However, coalition agreements, the party platform or (rarely) a resolution of the whole party group constrain the spokesperson’s room for manoeuvre.

In addition to the power of significantly defining the party’s position, Schöne (Reference Schöne2010) describes that, in exchange for the privileges those policy speakers have (e.g., media attention), they are expected to strictly represent their party group’s position and not dissenting opinions of their own. A recent example illustrates possible role conflicts: Whereas his party group was sceptical, the Social Democratic spokesperson for defence policy, Fritz Felgentreu, supported the claim of its coalition partner to purchase armed drones for the German military. After his party had postponed a decision once again, he resigned from office in 2020 and explained his decision on Twitter as follows (own translation):

Consequently, both the spokespersons’ greater influence on the decision-making process and their stronger attachment to the majority position leads to expect the following:

Many studies find that defection probabilities differ by policy issues (e.g., Skjæveland, Reference Skjæveland2001; Stecker, Reference Stecker2015; Bergmann et al., Reference Bergmann, Bailer, Ohmura, Saalfeld and Sieberer2016). However, this issue-based approach is only an approximation of where those differences actually stem from: a varying salience of the respective issue either for the party’s brand name (Traber et al., Reference Traber, Hug and Sciarini2014) or the electorate (Ohmura, Reference Ohmura2014). Accordingly, we expect that the importance of an issue conditions the explanatory power of division of labour as a pathway to unity. We focus on two dimensions of importance: the type of the vote and issue salience.

Not every vote is equally consequential. Owens (Reference Owens2003) argues that the politics of party unity is affected by the type of document which is put to the vote. In votes that are crucial for the functioning of the government (e.g., budget), party unity is expected to be higher than in less momentous votes (Rahat, Reference Rahat2007). According to Stecker (Reference Stecker2015), the vote type moderates also the determinants of unity, and he differentiates between legislative and non-legislative matters. Whereas the former exerted direct policy implications through changes in legal rules, the latter had a more symbolic value by being political expressions of intent without direct consequences – although the documents vary in importance within those categories as well.

Beyond vote type, the literature on issue ownership assumes that topics are not equally important to parties, their activists and voters (Budge, Reference Budge2015). This connection between parties and particular issues affects their strategies in election campaigns (Wagner and Meyer, Reference Wagner and Meyer2014), legislative agenda-setting (Green and Jennings, Reference Green and Jennings2019) and in government (Jensen and Seeberg, Reference Jensen and Seeberg2015). Since MPs are not only agents of their constituency but, primarily, of their parties whose success decides, for the most part, on their electoral and other career-related fortunes, the connection between parties and issues might also affect its MPs. Consequently, we expect a behavioural difference between high- and low-salience topics. The latter include those hardly covered in the election manifesto or technical issues difficult to grasp without specific policy knowledge (Andeweg and Thomassen, Reference Andeweg and Thomassen2011). Those topics are unlikely to be politicized since they do not relate to the party’s identity or the core interests and values of its voters. In such a setting, less constrained by the party manifesto and without creating facts by legislation, the policy-making process leaves enough room for parliamentary actors to substantially influence those policies. Accordingly, we expect division of labour to be particularly effective here, with lower defection rates of committee members.

In contrast, especially when consequential or salient issues are on the floor, parties must reckon with their actions being closely watched by voters. Electoral research has shown that voters indeed react to MPs’ legislative behaviour (Papp and Russo, Reference Papp and Russo2018; Wagner et al., Reference Wagner, Glinitzer and Vivyan2020; Duell et al., Reference Duell, Kaftan, Proksch, Slapin and Wratil2023). Disunity as a negative valence signal endangers the party’s issue ownership and, for government parties, their policy-making capability and will ultimately be punished by voters (Greene and Haber, Reference Greene and Haber2015; Lehrer et al., Reference Lehrer, Stöckle and Juhl2022). Accordingly, the party group leadership is inclined to use the whip to a stronger extent during consequential and salient votes in order to prevent harm to the party (Owens, Reference Owens2003). Consequently, for legislators, party unity hinges much more on a trade-off between their preferences, the interests of their constituency (Stiller, Reference Stiller2023) and the incentives of party discipline. In such a setting where parties and MPs have a clear opinion, taking voting cues from policy specialists becomes largely meaningless (Andeweg and Thomassen, Reference Andeweg and Thomassen2011; van Vonno, Reference van Vonno2016). Moreover, it is less necessary since intra-party preferences are more homogeneous than on less salient issues (Sieberer, Reference Sieberer2010). What is more, for legislative matters and salient issues, policy formulation transfers from the bottom-up process through committees more into a top-down process driven by top representatives of the coalition parties and the government bureaucracy (Schindler, Reference Schindler2019). Possible conflicts are then resolved elsewhere, which further limits the scope of committee members for shaping the party line. This leads to another two conditional hypotheses:

Data and methods

The propositions are tested quantitatively using data from the German parliament. First, the Bundestag is regarded as a ‘party group parliament’ (Ismayr, Reference Ismayr2012), i.e., a parliament with powerful party groups which dominate parliamentary work. Second, it is considered a ‘working parliament’ with a high degree of issue specialization among the MPs (Steffani, Reference Steffani1979; Lord, Reference Lord2018) and with the most policy-influential committees in Western Europe (Zubek, Reference Zubek2021). Therefore, Germany is a suitable (and certainly a most-likely) case for an argument based on the nexus between parties and committees. However, the findings should be generalizable to other parliaments in Europe with strong parties and at least a moderate influence of committees (and preparatory bodies of the party groups) on parliamentary decisions.

We collected data on all roll-call votes in the most recent 19^th parliamentary term (2017–2021). Despite the discussion on their representativeness and its consequences for the observed level of unity (Ainsley et al., Reference Ainsley, Carrubba, Crisp, Demirkaya, Gabel and Hadzic2020; Yordanova and Mühlböck, Reference Yordanova and Mühlböck2015), roll-call votes are the only source of individual-level voting behaviour for our case. Free votes, i.e., votes where MPs were released from party discipline, are, in principle, not restricted to morality policies but could be tactically imposed by the party group leadership if the party line is difficult to enforce (Ohmura and Willumsen, Reference Ohmura and Willumsen2022). In the legislative term under study, three unwhipped votes were held on a reform of the organ donation system and, thereby, a classical issue of conscience. Since the determinants of MPs’ behaviour systematically differ between whipped and unwhipped votes on morality policy issues, an exclusion of the latter from the sample is justified (see Mai et al., Reference Mai, Link and Engler2023 on MPs’ voting behaviour in those votes). Finally, the models consist of 241 roll-call votes on various kinds of documents (e.g., bills, amendments, motions or resolutions).

The unit of analysis is an MP’s voting decision in a single recorded vote. This highly disaggregated data structure is necessary given that the main independent variable, ‘own committee: at the time of the vote’, measures dichotomously whether an MP votes on a document that has been deliberated in a committee he/she belongs to. Consequently, this variable varies both on the MP and the vote level – such variance could not be explored if aggregated unity scores for MPs or votes were used. Initially, this variable is coded 0 for all votes on motions that were not assigned to at least one committee or, in the case of amendments, are not related to a bill assigned to a committee. This concerns about a quarter of all votes. For the remaining motions, on the vote level, we only consider the leading (federführend) committee since only in those committees, a thorough discussion takes place, possibly including expert hearings, and a voting recommendation for the plenary is resolved (§63 Bundestag rules of procedure). Other committees having only an advisory function in the deliberations are not considered in the main analysis. On the MP level, we only take ordinary committee memberships into consideration for our main models. Even though most MPs are deputy members of additional committees, they only attend their meetings when ordinary members are absent. More importantly, those deputy members are usually not involved in the discussions within the intra-party working groups. Consequently, our policy-shaping hypothesis does not apply to deputy members. In robustness checks, we will explore how the results change if those two restrictions are relaxed (Appendix A3). For now, at the descriptive level, 5.4% of the observations are cases where MPs vote on a document that has been dealt with in their committee(s).

For the variable ‘own committee: before the time of the vote’, the ordinary membership in the respective leading committee during the past two legislative terms, as well as in the current term until the day before the vote, is counted. We chose two past terms as a cut-off point because we assume that policy knowledge and connections to the former working group shrink the longer an MP is not a member anymore. In order to disentangle the effects of past and current membership, the variable is coded 0 if an MP is still a member of the respective committee. The dichotomous variable ‘policy spokesperson (all subjects)’ takes value 1 if an MP at the time of the vote leads a working group of the parliamentary party which is related to the work of one or more committee(s) – irrespective of whether the vote is pertaining to those issues. If the party has elected deputy group leaders who are responsible for certain (broader) topics, they are counted as spokespersons as well since they are actively involved in settling conflicts between different working groups and, as a connection between the leadership and working level of the parliamentary party, have significant powers in the decision-making process (von Oertzen, Reference von Oertzen2006). To compare the spokespersons’ behaviour across votes, we interact this variable with the variable ‘own committee’ in model 3. For investigating the conditional hypothesis 4, we dichotomized the votes by the respective documents into legislative (bills and amendments) and non-legislative matters (various kinds of motions and resolutions), following Stecker (Reference Stecker2015). In order to test H5, we interacted committee membership with issue salience for the MP’s party. We draw on Manifesto Project data (Lehmann et al., Reference Lehmann, Burst, Matthieß, Regel, Volkens, Weßels and Zehnter2022) which quantifies the percentage of ‘quasi-sentences’ a party dedicates to one of several dozens of issues in its election programme. Its focus on relative issue emphasis aligns with the assumptions of saliency theory (Budge, Reference Budge2015) and thus renders it the most suitable data source for our purposes. For ‘issue salience’, we summed up all manifesto items that are linked to the committee’s jurisdiction (irrespective of the party’s position on the issue). For the main analysis, we accept the huge differences among the salience scores since they represent the overall importance of the topics for the parties. Another option, which we pursue in the robustness checks, is to z-standardize the salience scores among the parties. The result would indicate then which party emphasizes a topic to an above- or below-average extent – irrespective of whether it is generally an important topic. The salience variables were coded ‘missing’ if no manifesto items fit to the committee’s jurisdiction. This reduces the number of observations in model 5 compared to models 1–4.

In order to isolate the hypothesized effects, we control for other factors that affect MPs’ defection rates. Most of them are approximations of the varying effectiveness of party discipline – since the national-level party leadership is the major principal for progressively ambitious MPs (Sieberer and Müller, Reference Sieberer and Müller2017). To capture electoral incentives, we include whether an MP was elected in the nominal or list tier of Germany’s mixed-member electoral system (Sieberer, Reference Sieberer2010). Especially, list candidates are punished by party selectorates for dissenting votes through less promising list positions (Schmuck and Hohendorf, Reference Schmuck and Hohendorf2022), whereas MPs, as individual candidates, tend to profit from vote defections, especially if justified with representing voter concerns (Duell et al., Reference Duell, Kaftan, Proksch, Slapin and Wratil2023). We also include dichotomous variables measuring whether an MP holds an executive office (chancellor, minister, junior minister) or another important parliamentary office (party group leadership, whip, Bundestag presidency, committee chairFootnote ¹ ) to account for higher disciplinary pressures (Zittel and Nyhuis, Reference Zittel and Nyhuis2019). Moreover, we consider parliamentary experience which was regarded as a proxy for MPs’ parliamentary socialization (Delius et al., Reference Delius, Koß and Stecker2013), policy influence (Tavits, Reference Tavits2009), policy convergence with one’s party group (Saalfeld, Reference Saalfeld1995) or expertise within ‘their’ policy area (von Oertzen, Reference von Oertzen2006). MPs’ gender and age are also included especially since recent studies detect, under particular circumstances, lower defection rates for women MPs (Clayton and Zetterberg, Reference Clayton and Zetterberg2021; Dingler and Ramstetter, Reference Dingler and Ramstetter2021). Finally, parties differ in their defection rates – a pattern which is also reflected in our data. Whereas the Christian Democrats show an average defection rate of 0.58%, the Green MPs cast 2.86% of their votes against the party line (Appendix A2, figure 3). Those inter-party differences were attributed to, among others, party group size (Bergmann et al., Reference Bergmann, Bailer, Ohmura, Saalfeld and Sieberer2016), candidacy eligibility criteria (Rehmert, Reference Rehmert2020), intra-party democracy (Close et al., Reference Close, Gherghina and Sierens2019), party organizational strength (Tavits, Reference Tavits2012) or party family (Close, Reference Close2018). Since only one term is analysed and thus sufficient variance is lacking, we include party-fixed effects to account for possible party differences.

Our dichotomous dependent variable takes value 1 whenever an MP’s voting behaviour differs from the majority of their party group. We do not differentiate between different degrees of dissent (i.e., between voting ‘abstention’ or ‘no’ if the party votes ‘yes’) since, in both mentioned cases, the MP does not ultimately support the party line. If an MP does not attend a vote, we code this as a missing observation in the main analysis. Although some absences might be strategically motivated in order not to dissent visibly from the party line (Font, Reference Font2020), they probably mostly have non-political reasons (e.g., time constraints, illness). Due to the dichotomousness of our dependent variable and a data structure that is clustered into multiple voting occasions of MPs, we estimate logistic panel regressions with standard errors clustered by MP.

Details on the measurement, data sources and descriptive statistics of all variables are listed in Appendix A1 (Table 1). Distributional graphs show that the defection rates vary strongly both among MPs and votes (Appendix A2, figures 1+2). Whereas about 50% of the MPs and 22% of the votes do not exhibit a single defection, some MPs have a much higher propensity to defect (up to 27%). Among the 20 votes with the highest defection rates (up to 10%), there are numerous decisions on military missions, but also on measures against the pandemic and other domestic policies (Appendix A2, Table 3).

Table 1. Results of the logistic regression analysis

Logit coefficients are displayed, with standard errors in parentheses.

Levels of significance: *p < 0.05, **p < 0.01, ***p < 0.001.