Online Appendix

Table A1: Partisan Identity Items in Original Language in Each Country

Netherlands Sweden United Kingdom

Als ik over deze politieke partij praat, dan zeg ik ‘wij’ in plaats van ‘zij’.

När jag pratar om partiet så säger jag oftast ”vi” snarare än ”de”.

When I speak about this party, I usually say “we” instead of “they”.

Ik ben geïnteresseerd in wat andere mensen van deze partij vinden.

Jag är intresserad av vad andra tycker om partiet.

I am interested in what other people think about this party.

Als mensen kritiek leveren op deze partij, dan voel ik mij persoonlijk beledigd.

När andra kritiserar partiet känns det som en personlig förolämpning.

When people criticize this party, it feels like a personal insult.

Ik heb veel gemeen met andere mensen die deze partij steunen.

Jag har mycket gemensamt med andra anhängare av partiet.

I have a lot in common with other supporters of this party.

Als deze partij het slecht doet in een poinie peiling, dan word ik daar chagrijnig van.

Om partiet fått dåliga siffror så är min dag förstörd.

If this party does badly in opinion polls, my day is ruined.

Als ik iemand ontmoet die deze partij steunt, dan voel ik me net die persoon verbonden.

När jag träffar anhängare av partiet så känner jag samhörighet.

When I meet someone who supports this party, I feel connected.

Als ik over deze partij praat, dan verwijs ik ernaar

När jag pratar om partiet säger jag

When I speak about this party, I refer to them as

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als ‘mijn partij’. ”mitt parti”. “my party”.Als mensen positief praten over deze partij, dan geeft me dat een goed gevoel.

När någon berömmer partiet så får det mig att må bra.

When people praise this party, it makes me feel good.

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Figure A1: Partisan Identity Option Response Function, Netherlands

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Netherlands Traditional PID Strength Item

Figure A2: Partisan Identity Option Response Function, Sweden

4

Sweden Traditional PID Strength Item

5

Figure A3: Partisan Identity Option Response Function, United Kingdom (Wave 3)

6

United Kingdom Traditional PID Strength Item (Wave 3)

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Figure A4: Distribution of Information across Partisan Identity Strength, Netherlands

Note: The distribution of information was calculated using an unconstrained graded response model. Information in the IRT parlance is a measure of reliability, indicating how much information the partisan identity scale provides for various levels of partisan identity strength, especially when they are in proximity to one another. Information levels are a function of standard error estimates.

Figure A5: Distribution of Information across Partisan Identity Strength, Sweden

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Note: The distribution of information was calculated using an unconstrained graded response model. Information in the IRT parlance is a measure of reliability, indicating how much information the partisan identity scale provides for various levels of partisan identity strength, especially when they are in proximity to one another. Information levels are a function of standard error estimates.

Figure A6: Distribution of Information across Partisan Identity Strength, UK (Wave 3)

Note: The distribution of information was calculated using an unconstrained graded response model. Information in the IRT parlance is a measure of reliability, indicating how much information the partisan identity scale provides for various levels of partisan identity strength, especially when they are in proximity to one another. Information levels are a function of standard error estimates.

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Table A2: Total Information Provided by the Partisan Identity Scale Items and the Traditional Partisan Strength Item

Total InformationNetherlands Sweden United

KingdomWhen I speak about this party, I usually say “we” instead of “they”.

3.94 3.45 4.54

I am interested in what other people think about this party.

4.00 3.39 3.40

When people criticize this party, it feels like a personal insult. *

4.49 3.39 5.93

I have a lot in common with other supporters of this party.

5.81 4.09 5.11

If this party does badly in opinion polls, my day is ruined.

3.76 3.20 5.04

When I meet someone who supports this party, I feel connected with this person. *

9.11 6.56 8.36

When I speak about this party, I refer to them as “my party”. *

6.07 5.94 8.71

When people praise this party, it makes me feel good. *

8.48 6.94 10.53

Partisan Identity Scale 45.65 36.95 51.61Short Partisan Identity Scale 28.14 22.83 33.533pt. Partisan Strength Item 2.87 3.04 2.80Partisan Identity and 3pt. Partisan Strength Item

48.52 39.99 54.41

Note: Numbers represent the amount of information provided by each item in the scale, the overall scale, the short scale as well as the traditional partisan strength measure. Calculations are based on an unconstrained graded response model.

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Table A3: Scalar Invariance Fit Measures (dropping item-by-item)

Model Chi-Square

Degrees of

FreedomTLI CFI RMSEA

All Items 3788.038 92 0.982 0.981 0.105

Excluding Item 1

3191.385 71 0.982 0.979 0.110

Excluding Item 2

3209.592 68 0.983 0.981 0.112

Excluding Item 3

3327.549 74 0.982 0.979 0.110

Excluding Item 4

3308.097 71 0.981 0.979 0.112

Excluding Item 5

3341.824 71 0.982 0.980 0.112

Excluding Item 6

3133.674 71 0.978 0.975 0.109

Excluding Item 7

3382.176 71 0.977 0.974 0.113

Excluding Item 8

2017.446 71 0.984 0.983 0.087

Note: Numbers illustrate changes in various fit measures of the scalar invariance model as each item in the partisan identity scale is excluded from the model.

Table A4: Ideological Intensity Scales

Dutch Parliamentary Election Studies 2012

Item 1 Ideology self-placement item: “Where would you place yourself on the scale below, where 0 means left and 10 means right?”

Mean value and Standard Error (unfolded)

0.52 (0.20)

Mean value and Standard Error (folded)

0.30 (0.26)

Correlation with Partisan Identity Scale (folded)

0.24

Correlation with 3pt. Partisan Strength Measure (folded)

0.29

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Swedish Citizen Panel 2013Item 1 Proposal: Reduce income

differences in society.Item 2 Proposal: Reduce Swedish

foreign aid to developing countries.

Item 3 Proposal: Increase unemployment benefits.

Item 4 Proposal: Lower taxes.Item 5 Proposal: Reduce the public

sector. Response categories Very good proposal, rather good

proposal, neither good nor bad proposal, rather bad proposal, very bad proposal

Average interitem covariance 0.81Scale reliability coefficient alpha (unfolded)

0.84

Mean value and Standard Error (unfolded)

0.36 (0.27)

Mean value and Standard Error (folded)

0.33 (0.21)

Correlation with Partisan Identity Scale (folded)

0.18

Correlation with 3pt. Partisan Strength Measure (folded)

0.17

British Election Study Online Panel Wave 4

Item 1 Left- Right value 1: Government should redistribute incomes.

Item 2 Left- Right value 2: Big business takes advantage of ordinary people.

Item 3 Left- Right value 3: Ordinary working people do not get their fair share.

Item 4 Left- Right value 4: There is one law for the rich and one for the poor.

Item 5 Left- Right value 5: Management will always try to get the better of employees.

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Response categories Strongly disagree, disagree, neither agree nor disagree, agree, strongly agree

Average inter-item covariance 0.58Scale reliability coefficient alpha (unfolded)

0.86

Mean value and Standard Error (unfolded)

0.71 (0.21)

Mean value and Standard Error (folded)

0.50 (0.30)

Correlation with Partisan Identity Scale (folded)

0.17

Correlation with 3pt. Partisan Strength Measure (folded)

0.15

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Table A5: Determinants of In-Party Vote and Political Participation by CountryIn-Party Vote Political

ParticipationNetherlands 1 2 3 4 5 6

Party Identity 2.64 (0.24) -- 1.16

(0.27)0.16

(0.01) -- 0.12 (0.01)

Partisan strength --- 1.64 (0.10)

1.39 (0.12) --- 0.06

(0.00) 0.04 (0.00)

Ideological intensity 1.13 (0.13)

0.87 (0.13)

0.82 (0.13)

0.02 (0.00)

0.02 (0.00) 0.01 (0.00)

Education 0.43 (0.14)

0.27 (0.14) 0.27 (0.14) 0.09

(0.00)0.08

(0.00) 0.08 (0.00)

Gender (Female) -0.11 (0.07)

-0.07 (0.07)

-0.06 (0.07)

-0.03 (0.00)

-0.03 (0.00) -0.03 (0.00)

Age 0.08 (0.02)

0.06 (0.02)

0.05 (0.02) 0.00 (0.00) 0.00

(0.00) 0.00 (0.00)

Employed 0.06 (0.07)

0.07 (0.08) 0.09 (0.08) 0.00 (0.00) -0.00

(0.00) 0.00 (0.00)

Constant -1.01 (0.16)

-.98 (0.18)

-1.05 (0.18)

-0.01 (0.01)

-0.00 (0.01) -0.01 (0.01)

Pseudo R2 squared/ R2 0.06 0.08 0.09 0.08 0.07 0.08N 3,793 3,793 3,793 3,804 3,804 3,804

SwedenParty Identity 3.55

(0.75) -- 2.32 (0.85)

0.33 (0.06) -- 0.23 (0.07)

Partisan strength --- 1.88 (0.35)

1.33 (0.41) --- 0.18

(0.03) 0.10 (0.04)

Ideological intensity -0.03 (0.28)

0.01 (0.28)

-0.09 (0.28)

0.18 (0.03)

0.19 (0.02) 0.17 (0.03)

Education -0.31 (0.40)

-0.31 (0.41)

-0.31 (0.41)

0.17 (0.04)

0.16 (0.04) 0.16 (0.04)

Gender (Female) 0.39 (0.22)

0.28 (0.22) 0.34 (0.22) -0.00

(0.02)-0.00

(0.02) -0.00 (0.02)

Age -0.02 (0.08)

-0.01 (0.08)

-0.02 (0.08) 0.00 (0.00) 0.00

(0.00) 0.00 (0.00)

Employed 0.28 (0.24)

0.24 (0.24) 0.29 (0.24) 0.01 (0.02) 0.00

(0.02) 0.01 (0.02)

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Constant 1.66 (0.66)

1.68 (0.65)

1.38 (0.67) 0.13 (0.07) 0.14

(0.07) 0.11 (0.07)Pseudo R2 squared/ R2 0.04 0.04 0.05 0.11 0.10 0.11N 1,328 1,328 1,328 763 763 763

United KingdomParty Identity 3.26

(0.23) -- 1.83 (0.26)

0.28 (0.01) -- 0.23 (0.02)

Partisan strength --- 2.18 (0.12)

1.72 (0.14) --- 0.12

(0.00) 0.05 (0.01)

Ideological intensity -0.51 (0.11)

-0.53 (0.11)

-0.63 (0.12)

0.08 (0.00)

0.09 (0.00) 0.08 (0.00)

Education -0.19 (0.13)

-0.17 (0.14)

-0.19 (0.14)

0.08 (0.01)

0.08 (0.01) 0.08 (0.01)

Gender (Female) 0.12 (0.08)

0.19 (0.08)

0.20 (0.08)

-0.02 (0.00)

-0.02 (0.00) -0.02 (0.00)

Age -0.05 (0.03)

-0.04 (0.03)

-0.06 (0.03)

-0.02 (0.00)

-0.02 (0.00) -0.02 (0.00)

Employed 0.16 (0.09)

0.12 (0.10) 0.16 (0.10) -0.01

(0.00)-0.01

(0.00) -0.01 (0.00)

Constant 0.15 (0.26)

-0.51 (0.27) 0.05 (0.27) 0.05

(0.02)0.11

(0.02) 0.05 (0.02)Pseudo R2 squared/ R2 0.05 0.08 0.10 0.15 0.12 0.16N 3,338 3,338 3,338 3,395 3,395 3,395

Note: In-party vote was estimated using a logit model; political participation was estimated using an OLS regression. Bolded coefficients are significant at p < 0.05 in a two- tailed test. All variables are scaled from 0 to 1, except for age which is measured in decades.

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Table A6: Determinants of In-Party Voting and Political Participation in the U.K. (including leader evaluation)

In-Party Vote Political Participation

United Kingdom

Party Identity 1.66 (0.26) -- 0.24 (0.01) --Partisan strength -- 1.57

(0.14) -- 0.09 (0.01)Party Leader Evaluation

3.87 (0.21)

3.66 (0.21) 0.07 (0.01) 0.10 (0.01)

Ideological intensity -0.13 (0.13) -0.19

(0.13) 0.10 (0.00) 0.10 (0.00)

Education -0.18 (0.15) -0.20 (0.15) 0.08 (0.01) 0.08 (0.01)

Gender (Female) 0.09 (0.09) 0.16

(0.09) -0.02 (0.00) -0.02 (0.00)

Age -0.04 (0.03) -0.05 (0.03) -0.02 (0.00) -0.02

(0.00)Employed 0.18 (0.10) 0.16

(0.11) -0.01 (0.00) -0.01 (0.00)

Constant -1.70 (0.31)

-1.47 (0.31) 0.02 (0.02) 0.05 (0.02)

Pseudo R- squared/ R- squared

0.17 0.19 0.16 0.14

N 3,271 3,271 3,327 3,327Note: Logistic regression was used to analyze in-party vote; political participation is analyzed with OLS regression. Bolded coefficients are significant at p < 0.05 in a two- tailed test. All variables are scaled between 0 and 1 for ease of interpretation, except for age which is measured in decades.

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Table A7: Variation Explained by the Partisan Identity Scale At Each Level of Traditional Partisan Strength

In-PartyVote

Political Participati

onAmong “not very strong”

identifiersParty Identity 0.80

(0.43)0.09 (0.02)

Netherlands -0.37 (0.17)

0.01 (0.01)

Sweden 1.75 (0.38)

0.35 (0.02)

Netherlands X Party Identity

0.19 (0.71)

-0.04 (0.04)

Sweden X Party Identity 0.25 (1.94)

0.04 (0.12)

Constant 0.14 (0.29)

0.01 (0.01)

N 2,132 2,091Among “fairly strong” identifiers”Party Identity 1.86

(0.37)0.18 (0.02)

Netherlands -0.05 (0.19)

0.05 (0.01)

Sweden 1.53 (0.31)

0.39 (0.02)

Netherlands X Party Identity

-0.93 (0.53)

-0.08 (0.04)

Sweden X Party Identity 0.15 (1.04)

-0.05 (0.06)

Constant -0.10 (0.26)

0.00 (0.02)

N 4,239 3,870Among “strong” identifiersParty Identity 2.42

(0.61)0.39 (0.04)

Netherlands -0.21 (0.40)

0.13 (0.03)

Sweden 2.30 (1.19)

0.45 (0.05)

Netherlands X Party Identity

0.13 (0.82)

-0.22 (0.06)

Sweden X Party Identity 0.45 (2.85)

-0.00 (0.10)

Constant 0.66 (0.51)

-0.11 (0.04)

N 2,088 2,001

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Note: In-party vote was estimated using a logit model; Political Participation was estimated using an OLS regression. Control variables include: Ideology, education, age, gender and working status. Bolded coefficients are significant at p < 0.05 in a two- tailed test. All variables scale from 0 to 1 for ease of interpretation, except for age which is measured in decades.

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Table A8: Replication of Table 6, Short Four-Item Partisan Identity Scale.

In-Party Vote Political Participation1 2 3 4 5 6

4-Item Party Identity Scale

2.49 (0.19) --- 1.34

(0.22) 0.23 (0.01) -- 0.17

(0.01)Partisan strength --- 2.05

(0.12) 1.68 (0.14) --- 0.12

(0.00) 0.06

(0.01)Ideological intensity 0.23

(0.08) 0.08 (0.08)

0.00 (0.08)

0.08 (0.00)

0.08 (0.00)

0.07 (0.00)

CountryNetherlands -0.08

(0.09)-0.52 (0.08)

-0.27 (0.10)

0.05 (0.00)

0.01 (0.00)

0.05 (0.00)

Sweden 1.79 (0.18)

1.49 (0.18)

1.65 (0.21)

0.38 (0.01)

0.31 (0.01)

0.34 (0.01)

Country X Party IdentityNetherlands X Party Identity

0.32 (0.28) --- -0.05

(0.32)-0.10

(0.02) -- -0.09 (0.02)

Sweden X Party Identity 0.27 (0.66) --- 0.46

(0.73)0.03

(0.03) -- -0.04 (0.04)

Country X Partisan StrengthNetherlands X Partisan Strength -- -0.15

(0.16)-0.07

(0.18) -- -0.05 (0.01)

-0.02 (0.01)

Sweden X Partisan Strength -- -0.18

(0.37)-0.28

(0.42) -- 0.08 (0.02)

0.09 (0.02)

Education 0.15 (0.09)

0.03 (0.09)

0.04 (0.09)

0.09 (0.00)

0.09 (0.00)

0.09 (0.00)

Gender (Female) -0.01 (0.05)

0.05 (0.05)

0.05 (0.05)

-0.02 (0.00)

-0.02 (0.00)

-0.02 (0.00)

Age 0.03 (0.01)

0.02 (0.01)

0.00 (0.01)

-0.00 (0.00)

-0.00 (0.00)

-0.00 (0.00)

Employed 0.14 (0.05)

0.13 (0.06)

0.16 (0.06)

-0.00 (0.00)

-0.00 (0.00)

-0.00 (0.00)

Constant -0.39 (0.16)

-0.11 (0.16)

-0.41 (0.17)

-0.03 (0.01)

-0.03 (0.01)

-0.03 (0.01)

Pseudo R- squared/ R- squared 0.09 0.12 0.13 0.32 0.32 0.33

N 8,441 8,441 8,441 7,944 7,944 7,944Note: In-party vote was estimated using a logit model; political participation was estimated using an OLS regression. Bolded coefficients are significant at p < 0.05 in a two- tailed test. All variables are scaled from 0 to 1, except for age which is measured in decades.

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Table A9: Short Four-Item Partisan Identity Scale, In-Party Vote, and Political Participation In-Party Vote Political Participation

Netherlands 1 2 3 4 5 64-Item Party Identity

2.39 (0.21)

-- 1.12 (0.24)

0.12 (0.01)

-- 0.08 (0.01)

Partisan strength --- 1.64

(0.10)1.38

(0.12) --- 0.06 (0.00)

0.05 (0.00)

Ideological intensity

1.16 (0.13)

0.87 (0.13)

0.83 (0.13)

0.03 (0.00)

0.02 (0.00)

0.01 (0.00)

Education 0.48 (0.14)

0.27 (0.14)

0.29 (0.14)

0.09 (0.00)

0.08 (0.00)

0.08 (0.00)

Gender (Female)

-0.13 (0.07)

-0.07 (0.07)

-0.07 (0.07)

-0.03 (0.00)

-0.03 (0.00)

-0.03 (0.00)

Age 0.08 (0.02)

0.06 (0.02)

0.05 (0.02)

0.00 (0.00)

0.00 (0.00) 0.00 (0.00)

Employed 0.05 (0.07)

0.07 (0.08)

0.09 (0.08)

-0.00 (0.00)

-0.00 (0.00) 0.00 (0.00)

Constant -0.93 (0.18)

-0.98 (0.18)

-1.02 (0.18)

-0.00 (0.01)

-0.00 (0.01)

-0.01 (0.01)

Pseudo R- squared/ R- squared 0.06 0.08 0.09 0.07 0.07 0.08N 3,793 3,793 3,793 3,804 3,804 3,804

Sweden4-Item Party Identity

3.00 (0.65) -- 1.96

(0.72)0.22

(0.05) -- 0.11 (0.06)Partisan strength --- 1.88

(0.36)1.38

(0.40) --- 0.18 (0.03)

0.14 (0.04)

Ideological intensity

-0.01 (0.28)

0.01 (0.28)

-0.09 (0.28)

0.20 (0.03)

0.19 (0.03)

0.18 (0.03)

Education -0.24 (0.40)

-0.31 (0.41)

-0.26 (0.41)

0.17 (0.04)

0.16 (0.04)

0.16 (0.04)

Gender (Female)

0.37 (0.22)

0.28 (0.22)

0.32 (0.22)

-0.00 (0.02)

-0.00 (0.02)

-0.00 (0.02)

Age -0.02 (0.08)

-0.01 (0.08)

-0.02 (0.08)

0.00 (0.00)

0.00 (0.00) 0.00 (0.00)

Employed 0.29 (0.24)

0.24 (0.24)

0.30 (0.24)

0.01 (0.02)

0.00 (0.02) 0.01 (0.02)

Constant 1.79 (0.65)

1.68 (0.65)

1.46 (0.66)

0.16 (0.07)

0.14 (0.07) 0.13 (0.07)

Pseudo R- squared/ R- squared 0.04 0.04 0.05 0.09 0.10 0.10N 1,328 1,328 1,328 763 763 763

United

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Kingdom4-Item Party Identity

2.77 (0.20) -- 1.53

(0.22)0.23

(0.01) -- 0.18 (0.01)

Partisan strength --- 2.19

(0.12)1.77

(0.14) --- 0.12 (0.00)

0.06 (0.01)

Ideological intensity

-0.49 (0.11)

-0.53 (0.11)

-0.63 (0.12)

0.09 (0.00)

0.09 (0.00)

0.08 (0.00)

Education -0.17 (0.13)

-0.18 (0.14)

-0.19 (0.14)

0.08 (0.01)

0.08 (0.01)

0.08 (0.01)

Gender (Female)

0.12 (0.08)

0.20 (0.08)

0.21 (0.08)

-0.02 (0.00)

-0.02 (0.00)

-0.02 (0.00)

Age -0.04 (0.03)

-0.05 (0.03)

-0.06 (0.03)

-0.02 (0.00)

-0.02 (0.00)

-0.02 (0.00)

Employed 0.15 (0.09)

0.12 (0.10)

0.15 (0.10)

-0.01 (0.00)

-0.01 (0.00)

-0.01 (0.00)

Constant 0.34 (0.26)

0.53 (0.26)

0.17 (0.27)

0.07 (0.02)

0.11 (0.02)

0.07 (0.02)

Pseudo R- squared/ R- squared 0.05 0.08 0.09 0.14 0.12 0.15N 3,320 3,320 3,320 3,377 3,377 3,377

Note: In-party vote was estimated using a logit model; political participation was estimated using an OLS regression. Bolded coefficients are significant at p < 0.05 in a two- tailed test. All variables are scaled from 0 to 1, except for age which is measured in decades.

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