When Does Increasing Mobilization Effort Increase Turnout? Evidence from a Field Experiment on Reminder Calls

Published date01 November 2020
Date01 November 2020
Subject MatterArticles
American Politics Research
2020, Vol. 48(6) 763 –778
© The Author(s) 2020
Article reuse guidelines:
DOI: 10.1177/1532673X20935786
Scholars and practitioners working in the domain of cam-
paigns and elections share a growing interest in understand-
ing which Get Out the Vote (GOTV) mobilization tactics are
effective at increasing participation levels and for what rea-
sons. Work on the timing of mobilization efforts (Nickerson,
2007; Panagopoulos, 2011) finds that mobilization closer to
an election is more effective than earlier contact. The inter-
ventions in these studies typically involve a single mobiliza-
tion attempt. In real campaigns, however, citizens may be
inundated by multiple mobilization attempts. This has also
led to growing interest in an important question about the
marginal returns to increasing the intensity of mobilization
efforts: Do additional attempts at contacting voters increase
turnout beyond the effects of a single mobilization attempt,
and if so, why?
Despite increased interest, there are few experimental stud-
ies that address these questions. The limited work that exists
involves phone-based GOTV campaigns and offers mixed
findings. Early research found that a second mobilization
attempt had no effect on turnout relative to an initial attempt
(Green & Gerber, 2001). More recent research has reported
relatively large positive effects of a second round mobilization
attempt on turnout beyond an initial successful call when the
second call provides a reminder to vote (Michelson et al.,
2009). These competing findings, coupled with the relatively
small number of studies on the efficacy of reminder calls,
means there is theoretical and empirical uncertainty.
Existing experimental evidence seems to suggest that a
second round mobilization attempt occurring just prior to an
election is effective at increasing turnout beyond a successful
initial call, but only when the second call is a reminder call
(and perhaps only among those who state they intend to
vote). But the literature is unable to clarify, both theoretically
and empirically, why second GOTV attempts containing a
reminder to vote are effective at increasing turnout beyond
an initial and early GOTV attempt. Nor has it fully tested
whether these effects depend on a citizen’s prior interest in
Most succinctly, existing experimental designs test bundled
treatments and do not provide leverage to test alternative
causal theories explaining observed effects. Additionally,
scholars have observed the fact that reminder call effects in the
literature are from studies that condition on subjects saying
they intend to vote and have assumed that the effects would
935786APRXXX10.1177/1532673X20935786American Politics ResearchGerber et al.
1Department of Political Science, Institution for Social and Policy Studies,
Yale University, New Haven, CT, USA
2Independent researcher, Yale University, New Haven, CT, USA
3Travers Department of Political Science, University of California,
Berkeley, Berkeley, CA, USA
Corresponding Author:
Alan S. Gerber, Department of Political Science, Institution for Social and
Policy Studies, Yale University, 77 Prospect Street, PO Box 208209, New
Haven, CT 06520-8209, USA.
Email: alan.gerber@yale.edu
When Does Increasing Mobilization Effort
Increase Turnout? Evidence from a Field
Experiment on Reminder Calls
Alan S. Gerber1, Gregory A. Huber1, Albert H. Fang2,
and Catlan E. Reardon3
When does increasing mobilization effort increase turnout? Recent experiments find second calls containing a reminder to
vote increase turnout beyond an initial contact. We argue existing studies cannot explain why reminder calls are effective
because they test bundled treatments including a late mobilization attempt, a late mobilization attempt given earlier contact,
and potentially activating reciprocity established in earlier contact. We report results from a two-round voter mobilization
field experiment that allows us to isolate these different mechanisms. We find that reminder calls increase turnout by 1.2%
points among subjects contacted in an earlier attempt, but that enhancing reciprocity by providing a reminder call offer
during an early call does not increase turnout beyond a second call. Additionally, we fail to find heterogeneous effects of
reminder calls by stated preference for a reminder or by stated vote intention, suggesting certain mechanisms do not explain
the effects of reminder calls.
voter mobilization, reminder calls, reciprocity, field experiment
764 American Politics Research 48(6)
not exist in the absence of such conditioning, leading them to
emphasize theories about the role of baseline preferences
toward voting. However, these mechanisms are not tested in
existing studies; doing so would require directly and formally
testing whether there are heterogeneous second round
reminder call effects by subjects’ prior stated vote intentions.
In this article, we address these concerns and make sev-
eral important incremental contributions to the literature. We
design and analyze data from a phone-based GOTV experi-
ment conducted by a non-profit, non-partisan organization in
Colorado during the 2014 midterm election. In the experi-
ment, the number and timing of GOTV call attempts (an
early or late GOTV call) and the contents of the initial call
(specifically, whether a reminder call to vote is offered in a
first round call) are randomly manipulated. This experimen-
tal design provides leverage to isolate the causal effects of
varying the level of campaign effort as a function of the num-
ber of calls, varying the timing of calls, and varying the
degree to which an expectation of reciprocity is established
and primed by manipulating the behavioral interactions
occurring between the citizen and the campaign caller. Our
design also provides leverage to test whether attempting a
second round reminder call has heterogeneous effects by
subjects’ stated vote intention.
We report three main empirical findings. First, we find
that attempting a late GOTV call increases turnout by
approximately 1.2% points among subjects successfully
reached in an early GOTV call. This effect is smaller than
some estimates previously reported in the literature, but pro-
vides additional experimental evidence supporting the argu-
ment that second round GOTV calls are effective at increasing
turnout levels conditional on a successful initial contact (in
support of Michelson et al. (2009), but contra Green and
Gerber (2001)). Second, we do not find support for the reci-
procity mechanism. The effect of a second round call is just
as large when a reminder call is offered in an early call as it
is when an offer is not provided in an early call. Third, we
find that the effect of a late GOTV reminder call does not
vary by subjects’ stated vote intention. Thus, the result we
observe in this study leads us to downgrade our belief that
observed second round reminder call effects on turnout are
attributable to either reciprocity or a class of explanations
emphasizing the causative role of intrinsic motivations to
vote or statements about future vote intentions that are
enhanced by a subsequent reminder call.
Existing Research
Does increasing mobilization effort increase turnout? The
literature suggests that a second GOTV attempt providing a
reminder to vote increases turnout among those successfully
contacted in an earlier GOTV attempt. We briefly describe
three experimental studies relevant to this argument and
identify two important questions about existing research.
First, we show that existing work is unable to clarify why
observed effects exist because existing experiments involve
bundled treatments that could operate through multiple
hypothesized mechanisms. Second, we argue that existing
theoretical explanations are incomplete because they inade-
quately distinguish between theoretical mechanisms related
to GOTV contact generally, repeated GOTV efforts, or repeat
interactions with a specific group. We argue that these theo-
retical ambiguities should inform experimental design.
Reminder GOTV Calls Increase Turnout
Green and Gerber (2001, pp. 21–22) conducted the first field
experiment to assess whether increasing mobilization effort
increases participation by testing the effect of a second
GOTV call relative to a single GOTV call on voter turnout
among a sample of registered voters aged 18 to 30. One of
the factors in their original
factorial design randomly
assigned half of the subjects to be called on Monday, the
night before the election ( n= 674), and the other half to be
called on both Sunday and Monday (n=
). This manipu-
lation provides leverage to identify the effect of additionally
calling on Sunday versus only calling on Monday on turnout.
They find no effect of the second call on turnout relative to
one call: the estimated average intent-to-treat (ITT) effect of
attempting two calls is −0.3% points (s.e. = 2.4, not signifi-
cant) relative to a one call group mean turnout rate of 75%
and the Complier Average Causal Effect (CACE) of two
calls is −0.4% points (s.e. = 3.1, not significant; estimated
proportion of Compliers = .845) (Green and Gerber, 2001, p.
37, Table 7). This study is unusual in that both calls are timed
to occur proximate to the election and to each other.
In subsequent research, Michelson et al. (2009) argue
instead that multiple mobilization attempts taking the form
of targeted reminder calls increase turnout, and that this
effect is particularly pronounced among registered voters
who are reached by a phone canvasser and who state to the
canvasser that they intend to vote in the upcoming election.
Of the four studies reported by Michelson et al. (2009), two
are relevant to the substantive quantities of interest we inves-
tigate in this paper. We briefly review the design and findings
of each in turn.
In the first of these studies, Michelson et al. (2009) ana-
lyze data from a field experiment conducted in 2003 where
the Public Interest Research Group (PIRG) targeted regis-
tered voters aged 18 to 24. The study involved 2,817 sub-
jects, all of whom were targeted by PIRG in an initial round
of mobilization attempts using both phone and door-to-door
canvassers prior to Election Day. About half of subjects were
randomly assigned to a treatment group for whom PIRG
would attempt follow-up GOTV calls containing a reminder
to vote on Election Day.1 The remaining subjects were
assigned to a control group that received no follow-up call
from PIRG beyond the initial GOTV attempt. Michelson
et al. (2009) report that attempting a follow-up GOTV call on
Election Day increases turnout by 3.8% points ( p)

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