How did something considered misinformation on Thursday afternoon become common knowledge by Thursday night? 

The social mechanism that converts supposed misinformation into consensus is a topic that I have studied for many years. It involves something ubiquitous: the twin acts of misrepresenting what we know and what we want under perceived social pressures. As I explained in my book, Private Truths, Public Lies, such expressions of insincerity can do serious harm. The Biden-Trump debate and its aftermath provide a powerful case in point. Allow me to explain.

Until Thursday night’s CNN debate, a majority of Democrats were afraid to tell a basic truth in public—to say openly what they know about Joe Biden’s physical and mental health—for fear of emboldening Donald Trump or taking a position that may seem adverse to their party. So they kept their knowledge of his declining cognitive abilities private and, in public, conveyed impressions and issued reports at odds with their own senses. 

If they felt that a different candidate had a higher chance of beating Trump, with rare exception they kept that preference hidden and pretended to agree with the idea, repeated ad infinitum, that Biden remained the party’s best hope.