If epistemic individualism is an adaptation for collective deliberation, there must be some mechanism whereby the products of collective deliberation come to be accepted. Under many conditions at least, epistemic diversity must be ‘transient’ (Zollman 2010), and that, in turn, requires that we are apt to defer to others.

Deference is in fact ubiquitous.Footnote 10 Global warming ‘skeptics’ do not come to their beliefs unaided. Instead, they defer to others (Kahan 2015; Keller 2015). Conversely, liberals exhibit much higher levels of trust in science (Lewandowsky and Oberauer 2016). As a consequence, conservatives and liberals are disposed toward biased assimilation of evidence supporting and opposing their views (Corner et al. 2012). In part, biased assimilation is caused by asymmetrical deference: accepting views from some sources and not others.

Asymmetrical deference may be partially explained by reference to message content (Keller 2015). Because responding to AGW very likely requires limitations on free markets, it is highly unpalatable to strong supporters of markets. While ‘solution aversion’ (Campbell and Kay 2014) may explain some of the observed variance, conservative distrust of environmentalism (and support for unfettered markets) appears to arise from, rather than cause, the partisan split. Conservationism has historically been a strong current within conservative thought, and up until recently there was no partisan divide on the environment, either within congress or on the part of the general public (McCright et al. 2014).

While message content certainly influences the disposition to defer (Harris 2012), sensitivity to properties of the testifier sometimes trumps content. Israelis and Palestinians evaluated a peace proposal significantly more or less favorably depending upon whether it was presented as coming from Palestinian negotiators or Israelis (Maoz et al. 2002). Similarly, conservative and liberal attitudes to proposals for welfare policies were much more strongly influenced by whether the proposals were said to be supported by Democrats or Republicans than by the content of the proposals (Cohen 2003). In fact, sensitivity to properties of testifiers is able to trump content even when the content of the testimony is disagreeable to hearers. Corrections of false claims are effective when they come from sources that share the ideology of the hearer (Nyhan and Reifler 2013) and also when they come from sources that can be expected to find the claim they affirm contrary to their own ideological interests (Berinsky 2017). Thus, corrections of false claims about Obamacare, for instance, are effective for both conservatives and liberals when they come from conservatives; the former because they share the ideology of the source, and the latter because they know that the source is likely to find the claim unpalatable. These data are powerful evidence that content-based explanations are insufficient on their own to explain deference.

This sensitivity to testimony source emerges early in childhood (Clément 2010; Sperber et al. 2010). From a very young age, children show a preference for informants who show signs of being benevolent and competent; scrutiny of informers intensifies around the age of four (Mascaro and Sperber 2009). I suggest that this filter plays a significant role in explaining the different patterns of deference exhibited by conservatives and liberals. The disposition to defer to others with whom we share a political or religious outlook is continuous with the disposition to use benevolence as a cue to reliability; we are disposed to see those with whom we share a political outlook and/or a religious affiliation as those who are benevolent toward us and our interests (or, perhaps, the disposition to use benevolence as a cue to reliability is just a special case of a disposition to defer to those with whom we share values). The use of political and religious affiliation as a proxy for benevolence is adaptive, inasmuch as sharing our values is being disposed to promote the things we value and to oppose the things we disvalue. The use of competence as a proxy for reliability protects us against fools; the use of benevolence as a proxy protects us against exploitation (see Hahn et al. 2016 for a model of how update on testimony is normatively rational as a function of perceived reliability and benevolence of sources).

Both sides utilitze these proxies to filter testimony. Thus, conservative Americans come to their anti-consensus views on these topics in precisely the same way in which their liberal counterparts come to their views: by deferring to those they rightly take to be more knowledgeable than, as well as benevolent toward, them. But because the topic has come to be politicized, this disposition to defer ensures that they do not defer to (or their chains of deference will not bottom out in) groups of scientists who espouse a view contrary to theirs. Since opposition to AGW has come to function as a marker of group allegiance, affirming the ‘wrong’ view constitutes a signal of a lack of benevolence and thereby of reliability. That is, since the left has come to be identified strongly with a particular view on AGW, affirming that view is signalling support of a political set of values and thereby a lack of benevolence to conservatives.

Whereas for liberals chains of deference trace back to the relevant scientific experts, and therefore to properly constituted collective deliberation, conservatives’ chains of deference end in ‘merchants of doubt’ (Oreskes and Conway 2010), or maverick scientists. Conservatives do not defer to scientists, or to their think-tank intermediaries or more local representatives, because while these sources exhibit cues of competence they fail to pass tests for benevolence. Liberals are epistemically luckier: they are disposed to defer to the most competent individuals and institutions, because these individuals and institutions pass tests for benevolence as well as for competence. Liberals defer to sufficiently large groups of sufficiently expert deliberators to ensure that their beliefs have a high degree of warrant; conservatives defer to a much smaller group of genuine experts and their chains of deference trace back as much or more to non-experts.Footnote 11 These facts (which are outside the purview of the individuals at the end of each chain) entail that one set of beliefs is very much better warranted than the other. Biased assimilation may thus be individually rational, whether it leads toward better or worse warranted beliefs.Footnote 12

It should be emphasised that there is nothing about the mechanism that undermines the warrant of conservatives’ beliefs about climate change or evolution that entails liberal invulnerability to it. Liberal trust in science is currently very much higher than that of conservatives, reflecting a loss of trust in science by conservatives since the 1970s (Gauchat 2012). Parallel historical processes could undermine liberal trust in science. Such an erosion would leave liberals vulnerable to individually rational but unreliable epistemic deference.Footnote 13 At the present time, however, dissent from the scientific consensus seems more often to be seen from those who strongly endorse free market ideology (there are, however, two important issues on which dissent is bipartisan: GMOs and vaccination; Lewandowsky et al. 2013b). Interestingly, the negative correlation between endorsement of free market ideology and rejection of climate change is significantly stronger than the negative correlation between the first and rejection of other well-established facts, such as the fact that smoking causes lung cancer and that HIV causes AIDS (Lewandowsky et al. 2013a). That suggests that content-based mechanisms play an independent role in epistemic deference. Whether the conservative suspicion toward science generalizes to other domains of expertise is currently unknown.

It is a further, interesting, question how issues come to be politicised such that espousing the warranted view constitutes a signal of unreliability. One live possibility is that the merchants of doubt, driven by specific commercial interests, undertook a successful strategy of politicising climate science such that it would come to serve as a marker of political affiliation and thereby a cue for benevolence or its lack. A deliberate campaign aimed at identifying AGW with the political and social left may have brought about a situation in which espousing an attitude on the topic constitutes evincing a strong signal of political affiliation and thereby of possessing or lacking reliability. This hypothesis is empirically tractable, though to my knowledge the detailed historical work required to support it remains to be done.