Knowledge and Assertion


Explain it like I'm 5

Humans might spend more time talking than on any other social activity. One important reason we talk is to share information. This occurs so naturally and easily for most of us that we take it for granted. But, upon reflection, it raises questions. For example, when we share information, what rules unconsciously guide our behavior? Evidence from biology, psychology, and philosophy reveals at least one such rule. According to this rule, assertions should express knowledge.

Explain it in like 5 minutes

The dilemma of trust

cain's betrayal
Cain's betrayal is discovered.

Trust is a double-edged sword. By trusting others, we stand to benefit from their talents and cooperation. But trust also leaves us vulnerable to manipulation, mistreatment, and betrayal. In this respect, humans face a dilemma encountered by every social species: how can we reap the benefits of trust while minimizing the downside of vulnerability?

This fundamental dilemma occurs in many areas of social life throughout the animal kingdom, including communication. Animals benefit from the tremendous amount of useful information that others can provide. But individuals can benefit from sending dishonest messages, such as false predator alarm calls in bird species, or lies among humans. If dishonesty occurs too much, then messages will eventually be ignored and, sadly, useful communication will cease.

How can this dilemma of trust in communication be solved?

Pointless and partial solutions

young george washington
Young George Washington confesses.

Some solutions are pointless. We could avoid being deceived by never trusting anyone. But this effectively gives up on communication to avoid its potential downside. Perhaps we could eliminate lying almost entirely by making it punishable by death. But this vicious regime would also frighten people into silence. Such extreme “solutions” throw the proverbial baby out with the bathwater.

Some solutions are only partial. At a certain point in human history, we started grappling with the problem by cultivating customs. We tell our children stories about boys who cry “Wolf!”, or future presidents who chop down cherry trees. And we develop formal institutions to discourage dishonesty, such as the convention of sworn testimony, codes of professional conduct, and “truth in advertising” laws. Although fables and legislation might help, they aren’t complete solutions. These tools developed in recent human history, long after our ancestors faced the dilemma of trust. But the dilemma is not limited to recent human history. It is as ancient as communication itself.

Natural solutions

Fortunately for us, we don’t have to start from scratch. Evolution has been crafting solutions for hundreds of millions of years. Biologists have discovered that other species keep dishonesty in check by instinctively following certain behavioral rules.

Sparrows use information-constrained signals to help solve the problem.

One such rule is to pay special attention to messages that only individuals with particular knowledge would produce. These are known as “information-constrained signals.” For example, a sparrow needs to distinguish other sparrows (“conspecifics”) who are invading its territory from those who innocently occupy neighboring territory. A sparrow accomplishes this based on whether the conspecific imitates the song the sparrow just sang (“song matching”), or sings a different song that the sparrow sang previously (“repertoire matching”). A neighbor would have had time to memorize other songs that the sparrow sang, but an invader would not. This makes repertoire matching an information-constrained signal of neighborhood. Accordingly, sparrows have come to instinctively rely on it when deciding how to respond to nearby conspecifics.

Monkeys use reputation to help solve the problem.

Another rule is social policing. Monkeys use predator alarm calls to warn group members of danger. With twenty pairs of eyes on the lookout, the monkeys benefit from increased safety. But sometimes an individual uses a false alarm call to scare others away from prized food. This might work initially, but the other monkeys notice what has happened and respond more skeptically to that individual’s future alarm calls. In effect, monkeys keep track of one another’s performance, and individuals can develop reputations as bad informants. The monkeys instinctively rely on reputation when deciding how to respond to a signal.

Wasps use punishment to help solve the problem.

To take another example, facial markings function as a sign of social dominance in some wasp species. A more complicated facial pattern correlates with greater fighting ability, so wasps with less complicated patterns often defer to those with more complicated patterns. This benefits the wasps by allowing them to settle disputes without risking injury in a physical fight. But facial patterns don’t settle all disputes: the wasps still fight sometimes. And if a particular wasp’s fighting ability falls short of what its facial pattern suggests, then the other wasps behave more violently towards it. In effect, wasps instinctively punish one another for sending inaccurate signals.

Human solutions

What behavioral rules do humans instinctively follow to help solve the dilemma of trust in communication?

human evolution
Evolution shaped human communicative instincts.

It turns out that we follow some of the same rules as birds, monkeys, and wasps. Like monkeys, we spontaneously keep track of people's performance, which results in them gaining reputations as good or bad informants. Like sparrows, we pay special attention to signals that bear the imprint of knowledge. Like wasps, we react negatively — though not, in the first place, violently! — when people make false assertions, or tell us things that they don't know are true.

young siblings
The instincts are evident early in development.

These behavioral patterns begin early in human development, remain evident through adulthood, and show up across different cultures. From a very young age, children spontaneously keep track of speakers' track records of accuracy and rely on that when making decisions. Young children base their social learning on judgments about what others know, and they learn better from speakers they view as knowing. Young children also cite people's knowledge, or ignorance, when explaining why they are good, or bad, at providing information.

As adults, familiar conversational habits reveal how strongly we associate knowledge and assertion. For example, we can naturally prompt someone to tell us what time it is by asking, "Do you know what time it is?" We can decline to answer a question on the grounds that, "I don't know." And we can challenge whether someone should have made an assertion by saying, "You don't know that."

The instincts are evident across cultures.

Many experiments have investigated this knowledge/assertion link. The central findings have been remarkably consistent. When deciding what someone should say, people pay close attention to what is true and what the speaker knows. False assertions should not be made, and when deciding what a speaker should assert, people are most strongly guided by what they think the speaker knows. People also prioritize information about what a speaker knows over information about the speaker's beliefs, evidence, or level of certainty. These patterns show up regardless of whether participants are young or old, male or female, or speak languages as different as American English and Korean.


A principal conclusion supported by all the evidence is that human communication bears some remarkable similarities to the communication systems of other species. Our human practice of assertion is implicitly governed by a socially-policed information constraint, a rule we instinctively follow. According to this rule, assertions should express knowledge.

How to learn more

Start with this brief review article. If that's not enough, you could read the book (published open access and freely available as a PDF).