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False narratives are powerful. Consider the Thanksgiving story most Americans grew up with—a feel-good tale of harmony and gratitude that neatly sidesteps the grim realities of colonization and violence.

Stories like these often get dismissed as harmless but they highlight something much more important: Falsehoods are seductive.They comfort us, reinforce what we want to believe, and feed our desire for moral outrage—especially when directed towards an outgroup we distrust, envy, or fear.

The point is: these stories aren’t confined to history books. Their influence ripples through our culture and politics, shaping how we see ourselves and the world.

Today, false narratives and misinformation seem to be spreading more quickly and widely than ever before. This raises a provocative question: Are political leaders—given their powerful role in shaping public discourse—responsible for what some call the “post-truth” era?

To find out, we used large language models to analyze decades of U.S. presidential speeches, tracking the prevalence of assertive falsehoods—claims made with certainty despite being untrue. The results are eye opening. Before the Reagan era, Republican and Democratic leaders used this rhetoric at similar rates. But after the Reagan era, a gap appears, with Republican leaders increasingly presenting falsehoods as facts compared to their Democratic counterparts. The trend isn’t about political leaders as a whole becoming more detached from truth—it’s about one party embracing this strategy.  

Donald Trump, however, is in a league of his own, standing out as an extreme outlier. His speeches included significantly more assertive falsehoods than any other U.S. President. Underscoring this finding, most of Trump’s statements during the 2024 Presidential Debate were classified as assertive falsehoods by our language models. His rhetoric isn’t just a continuation of past trends. It’s a seismic shift..

The risks of falsehoods

Misinformation and falsehoods leave real victims in their wake. During the COVID-19 pandemic, we analyzed geo-tracking data from millions of Americans to explore how political allegiances influenced public health behaviors. The results were concerning: political loyalty was six times more predictive of people’s actions during the pandemic than the actual risk of infection.

In Republican-leaning counties, skepticism about the virus led to fewer precautions, and ultimately, death rates that were significantly higher than expected. This reveals a sobering truth: Americans are living in politically constructed realities, where even life-and-death risks, like dying of COVID-19, are perceived through the lens of political allegiance.

The dangers don’t stop at public health. Our research uncovered a troubling connection between political leaders espousing falsehoods and subsequent political violence. Analyzing more than a century of data, we found that spikes in political violence often follow falsehoods presented as truths in presidential rhetoric. The January 6th Capitol insurrection is a stark example. Driven by Donald Trump’s baseless claims of a stolen election, the event highlights how political falsehoods can spill over into real-world chaos and harm.

Interventions that work

How can we counter these harmful forces? It’s unrealistic to assume we can stop leaders from spreading misinformation altogether. But can we at least limit the damage once these falsehoods reach the public? In a large study, we tested several strategies aimed at reducing the spread of misinformation on social media.

• Warning labels: Simple, clear overlays marking content as false were highly effective in curbing the sharing of fake news.
• Media literacy tips: Educational tools that help users distinguish fact from fiction worked as well as warning labels and increased the sharing of accurate information.
• Social norm nudges: Reminding users that most people value accurate information, had a modest but meaningful impact on reducing the spread of misinformation.

While these interventions are promising, scalability remains a major hurdle. Social media companies, driven by profit, often prioritize user engagement over truth. A troubling example is Meta’s recent decision to remove fact-checking labels, despite their proven effectiveness across political divides. Our research suggests that such moves are likely to deepen polarization, heighten political violence, and endanger lives.

Regulating misinformation must become a societal priority. We already regulate industries that pose significant risks—like food safety, transportation, and finance—because the costs of inaction are simply too high. Why should misinformation be treated any differently?

Philosopher Karl Popper famously warned of the "paradox of tolerance": if a tolerant society tolerates intolerance, it risks destroying itself. For too long, we’ve tolerated misinformation under the guise of free speech, and the consequences are now unmistakable—ranging from vaccine skepticism to the rise of extremist politics, such as alt-right movements.

There is an urgent need for systemic reforms to address the spread of falsehoods.

For a deeper dive into this critical issue, in the talk '' I explore the findings discussed above and share actionable strategies for combating the pervasive impact of misinformation.