  Deliberation among individuals online plays a key role in shaping the
opinions that drive votes, purchases, donations and other critical offline
behavior. Yet, the determinants of opinion-change via persuasion in
deliberation online remain largely unexplored. Our research examines the
persuasive power of $\textit{ethos}$ -- an individual's "reputation" -- using a
7-year panel of over a million debates from an argumentation platform
containing explicit indicators of successful persuasion. We identify the causal
effect of reputation on persuasion by constructing an instrument for reputation
from a measure of past debate competition, and by controlling for unstructured
argument text using neural models of language in the double machine-learning
framework. We find that an individual's reputation significantly impacts their
persuasion rate above and beyond the validity, strength and presentation of
their arguments. In our setting, we find that having 10 additional reputation
points causes a 31% increase in the probability of successful persuasion over
the platform average. We also find that the impact of reputation is moderated
by characteristics of the argument content, in a manner consistent with a
theoretical model that attributes the persuasive power of reputation to
heuristic information-processing under cognitive overload. We discuss
managerial implications for platforms that facilitate deliberative
decision-making for public and private organizations online.
