Communication and trust (COGSCI304)

The class is held on Tuesdays from 5 to 7pm, in room Théodule Ribot (29 rue d'Ulm, on the left after the first doors, to the right of a large empty space)

Master in Cognitive Science

PSL & Université Paris Descartes

Instructor: Hugo Mercier (

Semestre: S3, ECTS : 4; Number of hours: 24h CM; Prerequisites: none

Course taught in: English

1. Course description

The aim of this class is to give an overview of how people evaluate communicated information: How do we decide who knows best? How do we know who to trust? This class adopts an evolutionary and cognitive perspective. We will look at how evolution might have shaped the cognitive mechanisms with which we evaluate communicated information. The functioning of these mechanisms, in turn, shapes our culture: it helps explain which persuasion attempts are successful and which aren’t, which beliefs spread and which don’t, why we sometimes accept false beliefs and reject true ones. This class will look at different cognitive mechanisms—those that enable trust calibration, argumentation, emotional expressions—and how they develop in childhood. It will cover broad grounds in the types of communication events investigated, from everyday talk to propaganda, through rumors and fake news.


This class will rely heavily on an evolutionary perspective, making it more natural to follow for students who’ve taken Evolution and social cognition. However, those who have not can still take the class, but they should contact me (and read at least this

2. Learning outcomes

On successful completion of this course, students should be able to:

  • Understand the strengths and weaknesses of the various cognitive mechanisms we use to evaluate information;
  • Better understand how information spreads in a culture;
  • What are the main obstacles to mass persuasion attempts.

3a. Pedagogy, class organization, and homework

The class partly follows a classical lecturing template, partly a flipped classroom template, in which students read the material that would have been presented in a more typical lecture, and the time in class is spent discussing issues raised by the readings.

I welcome feedback on every aspect of the class, as well as requests for topics to cover. Comments and suggestions can be left anonymously there:

3b. Assessment

  • A short essay to be submitted by the next to last class (45%).
  • A presentation of this essay (15%).
  • Attendance and participation (this includes the questions to be sent before every class in relation with the readings) (40%).

3c. Readings

The main text we will rely on will be Not Born Yesterday: The Science of Who we Trust and What we Believe (In press at Princeton University Press). It’ll be emailed to the students who register in the class.

4. Course content


24/09 Class 1 Evolution of communication, epistemic vigilance

General principles of the evolution of communication: What is communication? How can it evolve? How can it remain stable?

Historical views on vigilance and credulity: Are people credulous? Who is more credulous? When are we most credulous? Why would we be credulous?

Epistemic vigilance: cognitive mechanisms aimed at evaluating communicated information. Why are they necessary? How are they organized?

Illustrations with the cases of subliminal influence and brainwashing.

08/10 Class 2 Emotional communication (taught by Guillaume Dezecache) SCHEDULED CHANGED

What are emotional signals? What do they express? How do they remain stable? How do we evaluate them? How do emotions affect crowds?

Readings: Not Born Yesterday, chapter 7; Dezecache, G., Jacob, P., & Grèzes, J. (2015). Emotional contagion: its scope and limits. Trends in cognitive sciences,

15/10 Class 3 Content evaluation

How do we decide what’s plausible? How do we integrate information that conflicts with our existing beliefs?

How the cognitive mechanisms that allow argumentation evolved. How these mechanisms work. Can argumentation allow us to make better decisions? When do people change their minds thanks to argumentation? What are the best conditions for argumentation to yield good outcomes? Why does polarization happen, in small groups and in societies?

Readings: Not Born Yesterday, chapters 1 to 4 (1 to 3 are simply to catch up on the first class), Mercier, H. (2016) The argumentative theory: Predictions and empirical evidence; To dig deeper: Wood et Porter 2016 The Elusive Backfire Effect Mass Attitudes' Steadfast Factual Adherence

22/10 Class 4 Trust: competence and benevolence

How do we know who knows best? How do we recognize that others are more competent than we are? How do we know when others have better informational access than we do?

How do we know who has our best interests at heart? How do we spot liars?

How do these mechanisms of trust calibration develop in childhood?

Normative theory of when we should trust the majority (Condorcet jury theorem). Factors to be taken into account (competence, honesty, independence).

How do we actually use majority information? How does this develop in childhood? When do we grant too much weight to the opinion of the majority?

Readings: Not Born Yesterday, chapters 5 and 6, to dig deeper: Harris et al 2017 Cognitive Foundations of Learning from Testimony

5/11 Class 5 Mass persuasion

Our mechanisms of epistemic vigilance should make attempts at large-scale persuasion difficult. Review of these attempts through the ages, and their outcomes—religious proselytizing, propaganda, advertising, political campaigns, etc.

Readings: Not Born Yesterday, chapters 8 and 9; DellaVigna et Gentzkow 2010 Persuasion- Empirical Evidence

12/11 Class 6 Rumors and reflective beliefs

Why are we interested in remote events, people, places that likely won’t affect us? Why does that provide impetus for the transmission of all sorts of inaccurate information? Why do we seem to accept all these inaccurate pieces of information? Why do we want to transmit them? What’s the cognitive status of many false beliefs? Why don’t we act as if we ‘really’ believed in them?

Readings: Not Born Yesterday, chapters 10 and 11; Dan Sperber, Intuitive and reflective beliefs.

19/11 Class 7 Absurd and extreme beliefs

Our mechanisms of epistemic vigilance evolved to function in environments that were vastly different from our current environments. This creates many opportunities for adaptive lags: situations in which our cognitive mechanisms have become potentially maladaptive due to rapid environmental change. These adaptive lags can explain why we sometimes develop extreme but false beliefs, make mistakes in who we believe to be our side, or in how competent we deem others to be.

Readings: Not Born Yesterday, chapters 12 and 13, Marquez 2013 A Model of Cults of Personality

26/11 Class 8 Mistaken deference and untrustworthy allies

Why do we sometimes defer to people who don’t know better? Why do we sometimes believe people are our allies when they aren’t?

Readings: Not Born Yesterday, chapters 14 and 15; Dan Sperber, The Guru effect

3/12 Class 9 Epistemic institutions

Why have such bizarre institutions as divination or the ordeal had such cultural success? Why do we have such poor intuitions about epistemic institutions (e.g. voting, deliberation, etc.)?

Readings: Mercier, H. (in press) A paradox of information aggregation: We do it well but think about it poorly, and why this is a problem for institutions, Boyer Why divination? Evolved psychology and strategic interaction in the production of truth; Bonus: Mercier, The cultural evolution of oaths, ordeals, and lie detectors”

10/12 Class 10 TBA

17/12 Class 11 Students’ presentations

Students present their work.

07/01 Class 12 Students’ presentations

Students present their work.

5. Course policies

Attendance. Regular attendance of, and punctual arrival at lectures are crucial to succeed in this course, and they are mandatory for all students registered for credit. This is important both for your individual success in this course, and for every other students’ success. Keep in mind in particular that, by arriving late, you are jeopardizing your own but also your classmates’ education by disrupting the flow of lectures. Practically speaking, if you are registered for credit then your grade will suffer from poor attendance or recurrent late arrivals. If you are not registered for credit, the same policy applies, though with different consequences: poor attendance or recurrent late arrivals may force us to ask you to stop auditing the course.

Participation. You are strongly encouraged to participate in lectures. This means asking deep and challenging questions, but also asking simple questions, asking for clarification, saying “I’m just not getting this, please explain it in some new way” or “I’m lost, can you remind me why we’re talking about this?” Although the class will be held in English, you can ask questions in French at any time.

You can contact the instructor via email.

Academic honesty policy Cheating will not be tolerated and may cost you your grade as well as have deeper repercussions in your academic career. The following is a non-exhaustive list of examples of what counts as cheating in this course: (i) signing on the attendance sheet without attending the class (e.g. signing and leaving, or signing for someone else); (ii) copying the essay of another student, with or without that student’s knowledge; (iii) copying portions of your essay from sources in the literature without giving them due credit.

Electronics The use of electronics (laptops, phones, etc.) will not be allowed in class (except if well justified, e.g. for medical reasons).