Two Decades of Polarization in American State Legislatures

# Two Decades of Polarization in American State Legislatures
### Boris Shor and Nolan McCarty
### University of Houston and Princeton University
### 2021-11-23

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# Introduction

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# TL,DR

- New data on individual- and aggregate-level state legislative ideology
  - Estimated using archives of roll call voting behavior and candidate surveys
- Polarization in state legislatures has only accelerated in the past decade

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- Asymmetric polarization
  - Both parties are polarizing, but at different speeds in different states
  - Democrats have been polarizing faster in more states than Republicans

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- Connection between opinion and legislative polarization is stronger in 2010s than 2000s

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# Data

- Our APSR paper was published in 2011
  - 1996-2009 with lots of missing data

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- We retooled and continued working
  - Revision control system
  - Professional programmer, Peter Koppstein, in charge of data project
  - New unique identifiers to disentangle data

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- Moar estimates at individual and aggregate levels
  - 61% more unique legislators (now 26,857)
  - 95% more chamber-years (now 2,687)

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# Descriptives

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# Change in Ideal Points Distributions

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# 1996 and 2020

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# 2020 for 50 states

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# Measures

- We measure polarization as the distance between party medians in a chamber-year
- Other measures are highly correlated with this one
  - Distance between means
  - Average pairwise distance between legislators
  - Standard deviation of ideal points

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# State: Average polarization for all years

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# National: Polarization trend by chamber

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# State: Chamber polarization variation in state trends

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# Region: Polarization trend

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# Region: Polarization trend in the West

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# Party median trends in the West

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# Long-Term Trends

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# State: Chamber polarization trends, 1977-2020

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# State: Chamber polarization trends, 1986-2020

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class: inverse, center, middle

# Asymmetric Polarization

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# Asymmetric Polarization

- NOMINATE measures of congressional polarization say that Republicans have been disproportionally responsible for the party divide

- What about the states?

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# National: Trend in party medians (levels), 1996-2020

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# National: Party medians (changes), 1996-2020

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# Region: Trends in party medians, 1996-2020

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# State: Differences in party median trends

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# Comparing Across States

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# Trend in Variance of Legislative Medians Across States

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# Implication

- States are considerably more heterogenous ideologically today as compared to 20 years ago
- Helps explain big policy divergence of states over time

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# State Opinion and Polarization

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# Replication of McCarty et al 2018

- Question: is there a connection between opinion and legislative polarization?
- In McCarty et al 2018, we combined state legislative data with Tausanovitch and Warshaw opinion data on 2000s districts
- Findings
  - Across-district polarization in 2000s districts is strongly related to state legislative polarization
  - Within-district polarization in 2000s districts is even more strongly related
- Replication finds even stronger relationships in 2010s districts

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# Opinion Heterogeneity Across Districts (Sorting)

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# Opinion Heterogeneity Within Districts (Divergence)

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# Future

- Collaboration with others to extend roll call data backwards in time
- Funding to continue collection moving forward
- 'Rosetta Stone' of unique identifiers to facilitate merging with other data sets
- Better infrastructure to share data

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# Thanks!

Questions?

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# Additional Slides

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# NPAT Responders per state

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# Correlations by Party Within States

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# Example mapping into NPAT space

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# NPAT Representativeness, by party

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# Polarization trend in the Northeast

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# Party median trends in the Northeast

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# Polarization trend in the South

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# Party median trends in the South

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# Polarization trend in the Midwest

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# Party median trends in the Midwest

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