Walton, Douglas N., 1997, Appeal to Expert Opinion: Arguments from Authority , University Park, PA: Pennsylvania ... is that when you get testimony from an authority that p, the authority ’s testimony is now the only reason ... difference between expert and novice testimony ? Do groups testify? And if so, how can we learn from a group’s ... This is because in the first case your belief is based on the testimony of a layman who is somewhat knowledgeable about the topic at hand, whereas in the second case your belief is based on the testimony of an epistemic authority (or, someone who is both your epistemic superior and an expert about the domain in question). (See Zagzebski 2012; Jäger 2016; Croce 2018; and Constantin & Grundmann 2020 for more on how the notion of an epistemic authority should be understood.) But how exactly should the difference between epistemic authorities and everyone else be accounted for? Broadly speaking, those working on the epistemology of authoritative testimony endorse one of two accounts: Preemptive Accounts and Non-Preemptive Accounts . Those who endorse a Preemptive Account of authoritative testimony accept Preemption : The fact that an authority… [testifies] that p is a reason for me to believe that p which replaces my other reasons relevant to p and is not simply added to them. (Zagzebski 2012: 107) The key idea here is that when you get testimony from an authority that p , the authority’s testimony is now the only reason that you have for believing p , i.e., any other reasons you may have had are now preempted in the sense that they no longer count for or against p . Proponents of the Preemptive Account, then, explain the difference between authoritative and non-authoritative testimony as follows: Authoritative testimony can provide you with a preemptive reason for belief, whereas non-authoritative testimony cannot. Instead of thinking about authoritative testimony as providing preemptive reasons for belief, proponents of the Non-Preemptive Account take an authority’s testimony that p to provide a very strong reason to believe that p , where this reason is to be added to, or combined with, all of the other reasons that you have related to the proposition in question. See Dormandy (2018) and Lackey (2018a) for defenses of Non-Preemptive Accounts. Graham (1997: 227) offers a different account of testimony based on conveying information, i.e., a speaker, S , testifies that p if, and only if, (i) S ’s stating that p is offered as evidence that p (ii) S intends that his audience believe that he has the relevant competence, authority, or credentials to state truly that p and (iii) S ’s statement that p is believed by S to be relevant to some question that he believes is disputed or unresolved (which may or may not be whether p ) and is directed at those whom he believes to be in need of evidence on the matter. (J. 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J., 1992, Testimony: A Philosophical Study , Oxford: Clarendon Press. doi:10.1093/0198235518.001.0001 –––, 1994, “Testimony, Observation and ‘Autonomous Knowledge’”, in Matilal and Chakrabarti 1994: 225–250. doi:10.1007/978-94-017-2018-2_11 Constantin, Jan and Thomas Grundmann, 2020, “Epistemic Authority: Preemption through Source Sensitive Defeat”, Synthese , 197(9): 4109–4130. doi:10.1007/s11229-018-01923-x Crisp, Roger, 2014, “Moral Testimony Pessimism: A Defence”, Aristotelian Society Supplementary Volume , 88: 129–143. Selby-Bigge (ed.), second edition, revised P. H. Nidditch (ed.), Oxford: Oxford University Press, 1978. –––, 1748 [1977], An Enquiry Concerning Human Understanding , London: A. Millar. Reprinted, Eric Steinberg (ed.), Indianapolis: Hackett Publishing Company, 1977. Insole, Christopher J., 2000, “Seeing off the Local Threat to Irreducible Knowledge by Testimony”, The Philosophical Quarterly , 50(198): 44–56. doi:10.1111/1467-9213.00166 Jäger, Christoph, 2016, “Epistemic Authority, Preemptive Reasons, and Understanding”, Episteme , 13(2): 167–185. doi:10.1111/j.1520-8583.2009.00174.x –––, 2011, “Skepticism about Moral Expertise as a Puzzle for Moral Realism”, Journal of Philosophy , 108(3): 111–137. doi:10.5840/jphil201110837 McMyler, Benjamin, 2011, Testimony, Trust, and Authority , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780199794331.001.0001 McMyler, 2013, “The Epistemic Significance of Address”, Synthese , 190(6): 1059–1078 McShane, Paddy Jane, 2018a, “The Non-Remedial Value of Dependence on Moral Testimony”, Philosophical Studies , 175(3): 629–647. doi:10.1111/j.1468-0114.1993.tb00355.x –––, 2010, “Experience and Testimony in Hume’s Philosophy”, Episteme , 7(1): 42–57. doi:10.3366/E174236000900080X Van Cleve, James, 2006, “Reid on the Credit of Human Testimony”, in Lackey and Sosa 2006: 50–70. doi:10.1093/acprof:oso/9780199276011.003.0003 Walton, Douglas N., 1997, Appeal to Expert Opinion: Arguments from Authority , University Park, PA: Pennsylvania State Press. 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People increasingly rely on social media for health information, despite substantial variability in information quality. We conducted a systematic review and meta-analysis examining whether people are sensitive to source expertise when evaluating and sharing online health claims, and if this depends on whether the source is an individual expert (e.g., a physician) or an expert group (e.g., a health organisation). Three searches across four databases identified 25 individual studies published across 22 papers. From these studies, we found that people were only marginally more persuaded by health experts than nonexperts, and that this effect was not moderated by expert type (individual vs. group). However, we found tentative evidence that experts are more persuasive when accompanied by congruent credibility cues, but that expertise can backfire when these cues are incongruent. Evidence regarding people's sensitivity to source expertise when sharing was mixed, reflecting heterogeneity in study designs. This review highlights several limitations in the current literature, indicating the need for further research that accounts for how both experts and nonexperts are perceived in online health contexts. We discuss implications and propose several avenues for future research and potential interventions.