The Ties that Bind Us: The Influence of Perceived State Similarity on Policy Diffusion

Date01 June 2021
Published date01 June 2021
Subject MatterArticles
Political Research Quarterly
2021, Vol. 74(2) 377 –387
© 2020 University of Utah
Article reuse guidelines:
DOI: 10.1177/1065912920906611
Since Walker’s (1969) article on policy diffusion, research
focusing on how policies spread across the United States
has often relied on a key assumption—geographic conti-
guity drives diffusion. Early diffusion literature argues
that policies spread more readily from state to state when
the states border each other or are in the same region
(Berry and Baybeck 2005; Gray 1973; Walker 1969).
Meta-analyses of the literature show that contiguity is
almost always included in diffusion models and is often a
predictor of policy adoption (Maggetti and Gilardi 2016).
Yet, there are plenty of examples of how contiguity
does not explain how policies travel across U.S. states.
The legalization of same-sex marriage is one example.
Massachusetts and Connecticut were the first two states
to legalize same-sex marriage in 2003 and 2008. Iowa,
however, halfway across the country, was the third
adopter in 2009. This is one of many examples of non-
contiguous policy adoption. Clearly, there is more at play
in how policies diffuse than just geographic proximity.
Recent research has challenged contiguity as a mea-
sure of diffusion and proposed alternative understandings
of policy adoption and innovation. Scholars are using
more sophisticated measures and methods to understand
diffusion beyond the role of contiguity (Desmarais,
Harden, and Boehmke 2015; Nicholson-Crotty and
Carley 2018; Pacheco 2012; Shipan and Volden 2012).
This more methodologically rigorous research has shown
that, while contiguity is relevant to understanding policy
diffusion, it is only a “good starting point” but is “overly
limiting” and “sometimes misleading (or even wrong)”
(Gilardi 2016). Despite this, scholars continue to include
contiguity as a one-size-fits-all variable in model
We propose using a new measure, perceived state
similarity, as a more sophisticated and versatile alterna-
tive to contiguity. In this study, we generate and use a
continuous measure of citizen perceptions of state simi-
larity to predict the diffusion of eighty-nine policies
adopted from 2012 to 2016. We find that perceived state
similarity is a strong predictor of dyadic policy similar-
ity. We also find that similarity remains a strong predic-
tor of diffusion when expanded to a larger set of policies
in a pooled event history analysis (EHA) from 1990 to
2016. We suggest scholars consider moving beyond
contiguity to understand relationships between states
when modeling policy adoption and innovation, and use
perceived state similarity as a way to understand inter-
state connections.
906611PRQXXX10.1177/1065912920906611Political Research QuarterlyBricker and LaCombe
1Warren Wilson College, Swannanoa, NC, USA
2The University of Iowa, Iowa City, USA
Corresponding Author:
Christine Bricker, Department of History & Political Science, Warren
Wilson College, 701 Warren Wilson Rd., Swannanoa, NC 28778,
The Ties that Bind Us: The Influence
of Perceived State Similarity
on Policy Diffusion
Christine Bricker1 and Scott LaCombe2
In this paper, we propose a new measure to understand policy connections between the states. For decades, diffusion
scholars have relied on the largely untested assumption that contiguous states are more similar than noncontiguous
states, despite evidence that similarity is more complex than geographic proximity. We use a unique survey of citizens’
perceptions of other states to construct a national network of similarity ties between the states. We apply this new
measure with a data set of state policy adoptions in a dyadic and monadic event history analysis and find that similar
state adoptions are a reliable predictor of policy innovation. We argue that perceived state similarity is a more
complete measure of how states look to each other than contiguity.
diffusion, similarity, policy, state politics, event history

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