Political scientists are increasingly adopting psychophysiological research modalities to investigate the biomarkers of political attitudes and behavior. A good deal of this research focuses on the sympathetic branch of the autonomic nervous system. This makes a good deal of sense as psychophysiologists have long associated the sympathetic nervous system (SNS) with the sorts of implicit emotional-cognitive processing theorized to underpin a range of political attitudes. This review assesses the literature examining the relationship between political attitudes and individual-level variation in SNS activation, especially in response to disgust/threat stimuli where non-physiological research provides the basis for a strong a priori hypothesis for the existence of such a relationship. The empirical record for this relationship proves to be mixed, with a number of studies supporting the base theoretical expectations, but failed replications and questions about what is actually being measured also raising questions about the generalizability of the findings.
Introduction
A considerable research literature suggests that political attitudes and behaviors are genetically, which is to say biologically, influenced. Well-powered analyses using a variety of methodological approaches—including twin studies, adoption studies, and genome wide association studies—converge on the inference that a non-trivial amount of variation in political beliefs and behaviors systematically maps onto genetic variation [1, 2, 3, 4]. While the evidence of biological influences on political phenotypes is persuasive, the specific downstream mechanisms explaining this link have only recently begun to be systematically investigated. Ideology is a complex social phenotype. Its heritable components are almost certainly polygenic in nature, and the biological mechanisms that presumably mediate between genes and political beliefs almost certainly interact with environmental influences in ways that are far from fully understood. How does biology actually influence political traits? Currently, the only honest answer to this question is that we are not completely sure.
While no comprehensive, universally accepted answer exists to this question, a rough theoretical model emerged over the past decade or so to guide investigations of the link between biology and ideology. Succinctly, this model assumes genetic variation leads to individual-level differences in the physiological systems that not only play a key role in extracting and processing information from the external environment, but also in generating automatic emotional and behavioral responses to a given environmental situation or stimuli. The essential idea is that genes build biological information processing systems, there is individual-level variation in those systems due to both genetic and environmental influences, these individual-level variations lead to differences in implicit emotional and cognitive responses to environmental stimuli, and those differences reflect physiologically instantiated predispositions that at least partially drive political preferences [5, 6, 7].
An obvious general hypothesis generated by this framework is that individual-level differences in physiological responses to particular stimuli should predict political beliefs and behavior. An initial tranche of mostly small-N studies focusing on the sympathetic branch of the autonomic nervous system (ANS) reported evidence of exactly such links, though the reported physiology/ideology relationships centered more on certain attitudes associated with social conservatism rather than ideology more broadly conceived. Those relationships, however, have not fully replicated in more recent, well-powered replications. This raises questions not just about the specific relationships tested, but the broader theoretical framework generating the hypotheses.
We review this research and, especially in light of recent replication failures, examine implications for future research. We suggest one way forward is to narrow both the theoretical and empirical approach, focusing on physiological responses to more narrowly targeted stimuli and how those responses do or do not predict more specific political attitudes.
The ANS as a basis to investigate Links between physiology and ideology
The primary purpose of the ANS is to maintain homeostasis between external and internal environments through the regulation and coordination of bodily functions like digestion, respiration, and cardiac activity. These regulatory functions are largely automatic and implicit and occur outside of conscious awareness. The ANS has two primary branches, the sympathetic nervous system (SNS; colloquially known as the ‘fight or flight’ system) and the parasympathetic nervous system (PNS; the ‘rest and digest’ system).
Most research targeting the ANS to investigate the physiological underpinnings of ideology has focused on the SNS. This is in no small part because of the availability of a well-validated measure of SNS arousal, electrodermal activity (EDA), that can be obtained reliably and relatively cheaply using little more than a bioamplifier and sensors capable of capturing skin conductance.
The SNS represents a good target for testing links between physiology and ideology because psychophysiologists have long associated SNS activity with various aspects of automatic emotional-cognitive processing [8]. Some of those processes are, prima facie, good candidates to map onto political attitudes and behaviors. For example, recent literature reviews suggest the attitudinal and behavioral differences between liberals and conservatives are systematic, are anchored in traits such as negativity bias and in-group versus out-group bias, and almost certainly have a neurobiological basis [9]. That neurobiological basis clearly encompasses the ANS because it is already associated with some of these traits. It is well established, for example, that negative stimuli evoke a greater SNS response than non-negative stimuli, and this includes stimuli such as negative news stories that are relevant to politics [10, 11, 12, 13]. So variation in skin conductance seems to reliably capture (among other things) individual-level variation in negativity bias, and individual-level variation in negativity bias is widely hypothesized to systematically co-vary with political beliefs. This is all consistent with the hypothesis that SNS response to particular types of negatively valenced stimuli will predict political beliefs.
Exactly such arguments have already been made in relation to specific categories of aversive stimuli, especially threat and disgust. Numerous studies using non-physiological (self-report) measures have repeatedly found disgust and threat sensitivity correlate with political attitudes [14, 15, 16, 17, 18, 19], and a meta-analytic review encompassing 134 samples from 16 countries concludes there is consistent evidence that both subjective perceptions and objective experiences of fearful or threating stimuli correlate with conservatism [20]. Theoretically, this relationship is assumed to be anchored in evolved implicit processes [21]. In short, there is consistent and persuasive evidence that variation in threat and disgust responses as captured by self-report batteries is predictive of political attitudes. As there is little doubt such stimuli also evoke SNS arousal [17,22], a clear physiology-based hypothesis is suggested, that is, that individual-level variation in EDA response to such stimuli should predict political beliefs. There now exists a fairly extensive literature focused on testing these sorts of hypotheses that, in effect, combine what is known about how responses to non-political stimuli map onto political beliefs, and how the SNS is known to respond to similar sorts of stimuli.
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