Truth-Default Theory: Why Believing Is the Normal State

After a scam, the question people ask themselves is almost always the same: how could I have believed that? It assumes that belief was the mistake, that a sharper person would have started from doubt. A body of deception research led by communication scholar Timothy Levine says the opposite. Starting from belief is what human communication normally looks like. The useful question is not why someone believed, but what would have had to happen for them to stop.

Truth-default theory, first set out in a 2014 journal article, holds that people tend to accept what others say unless something specific triggers suspicion. Studies since then, including recent work on voice phishing and email scams, suggest manipulators succeed largely by never pulling that trigger. That shifts attention from the person who believed to the moments where doubt could have been designed in.

The theory in brief

Levine’s paper in the Journal of Language and Social Psychology states its premise in one sentence: “The central premises of TDT are that people tend to believe others and that this ‘truth-default’ is adaptive.” The theory also argues that lie detection works better through the content and context of what is said than through body language, nervousness or other behavioural cues. That second claim runs against much popular advice about “spotting a liar.”

The third column is our application, not Levine’s. But it follows closely from the theory’s logic. If belief is the resting state, a manipulator’s job is not to overcome doubt. It is to keep the conversation in the zone where doubt never arises.

What the evidence shows

The truth-default is not just a thought experiment. In two experiments reported in Human Communication Research in 2019, Clare and Levine looked at what people spontaneously think about when listening to others. Absent prompting, participants “mentioned consideration of the veracity of other’s communication less than 10% of the time.” The authors wrote that the findings “supported the concept of a truth-default.” People were not deciding that speakers were honest; mostly, they were not considering the question at all.

An earlier set of experiments in the same journal, by Levine, Kim and Blair in 2010, tested the projected-motive idea: “Absent a perceived motive for deception, people will infer that a message source is honest.” As predicted, confessions, which carry no obvious motive to lie, were judged honest more often than denials. One of the three experiments used professional investigators, which matters for the idea that experts escape the default. They did not, in that study.

Studies that apply it to scams

The vishing study by Armstrong and colleagues concludes that “people begin vishing conversations in the truth-default state,” that certain parts of the call act as triggers and that others do not. Its practical recommendation is to address “targeted individuals’ lack of concern about seemingly innocuous requests.” A small sample of 24 limits how far the numbers generalize, but the pattern is striking. Callers who asked for something obviously sensitive set off suspicion. Callers who asked for small, harmless-sounding things did not.

The 2026 study by Li and Chua adds a twist. Phishing awareness, they found, “plays a dual role. While it deters users from responding, it also makes knowledgeable users less vigilant against messages perceived to be personally appealing.” Knowing about scams did not switch off the default; for messages that felt relevant, it may have made people more confident they would notice a problem. That echoes a theme in our article on what a 54-study phishing review really shows: susceptibility depends on the message and the moment, not on a fixed type of person.

Why the default is not a weakness

It is tempting to read all this as proof that people are naive. The theory’s claim that the default is adaptive points the other way, and the everyday logic is easy to see. Most of what people are told in a day is true. A colleague says a meeting moved; a neighbor says the road is closed; a parent says they called the pharmacy. Checking all of it would be exhausting and socially corrosive. A default of belief lets cooperation run at the speed daily life requires, and the occasional cost of being deceived is outweighed by the benefit of not treating everyone as a suspect.

That is why advice to “be suspicious of everything” fails in practice. It asks people to give up an adaptive habit in every interaction to catch a rare event. Our piece on security decision overload describes what happens when the number of things people are asked to scrutinize keeps growing: scrutiny gets spread thin, and the signal is lost.

Designing triggers instead of demanding doubt

If belief is the default, the better strategy is to make specific situations act as triggers automatically, regardless of how the person feels about the conversation. The theory’s logic and the vishing findings point the same way: people react to sensitive requests, so define which requests count as sensitive and attach a fixed response to them.

• Turn categories into triggers. A request for a one-time code, a password, remote access, a payment, a bank-detail change or a gift card triggers verification by rule. Nobody has to feel suspicious first.
• Watch for the innocuous build-up. The Armstrong study found that harmless-seeming requests did not lower trust. Training can name the pattern: a series of small, reasonable asks that leads toward a sensitive one.
• Supply the motive the scam hides. Projected-motive research says people trust when they see no reason for deception. Reminding staff and family that anyone asking for money or access has an obvious motive to lie restores the missing trigger.
• Make the check social, not personal. A rule that says “we always call back” lets someone verify without accusing the caller, which keeps the cost of doubt low.

This also reframes what awareness training is for. Teaching people that scams exist does little against a default that is not about knowledge. Teaching them a short list of situations that always get checked, and giving them a script to do it, works with the default instead of against it.

A note for anyone who was fooled

If you believed someone who turned out to be lying, you were using the same mental setting that lets you function in every other conversation of your day. The person deceiving you knew that and worked to stay inside it. That is not a character flaw to be ashamed of; it is what the research predicts. The more useful reflection is not “why did I trust?” but “was there a moment that should have counted as a trigger, and how can I make that moment automatic next time?” For families talking through what happened, our guide to keeping the conversation open without shame starts from the same place.

Sources

• Levine, Truth-Default Theory (TDT): A Theory of Human Deception and Deception Detection (Journal of Language and Social Psychology, 2014): central premises (abstract).
• Clare and Levine, Documenting the Truth-Default (Human Communication Research, 2019): spontaneous veracity consideration under 10%.
• Levine, Kim and Blair, projected motive model (Human Communication Research, 2010): motive and honesty judgments.
• Armstrong, Jones and Siami Namin, truth-default and vishing (Human Factors, online 2021): vishing honesty ratings and triggers.
• Li and Chua, Message involvement increases phishing susceptibility (Information & Computer Security, 2026): involvement, suspicion and the dual role of awareness.