It’s late 1941, and seven-year old Daniel Kahneman has stayed at a friend’s house past curfew. Before he hurries home through the empty streets of Nazi-Occupied Paris, he turns his sweater inside out, hiding the yellow Star of David sewn to the front.

When a German soldier in an SS uniform spots him on the street, and beckons, Daniel is terrified he’ll make out the yellow star through his sweater. But instead of arresting him, the soldier picks him up and embraces him. After putting him down, the soldier speaks to him in German, and with great emotion, opens his wallet to show him a little boy, about his age. The weepy Nazi then hands him money from his wallet, and sends him on his way.

That moment would shape the rest of Daniel Kahneman’s life.

As he’d say later, "I went home more certain than ever that my mother was right: people were endlessly complicated and interesting."

That moment of cognitive dissonance—the recognition that a Nazi soldier could be moved by paternal love even as he participated in the wholesale murder of other people’s children—planted the seed for one of the most revolutionary discoveries in modern psychology. The SS officer had made an associative error, seeing in Daniel’s face his own son rather than the Jewish child he was trained to despise. It was irrational, contradictory, and, as Kahneman would later understand, deeply human.

That childhood fascination with human contradiction eventually led Kahneman to psychology. After his family fled France in 1946, he studied at Hebrew University in Jerusalem and later earned his doctorate at Berkeley in 1961. By the late 1960s, he was back teaching at Hebrew University when a colleague named Amos Tversky gave a guest lecture in one of his seminars.

Tversky was everything Kahneman wasn't—a mathematical psychologist with an ego to match his brilliance. Where Kahneman was anxious, Tversky was confident. Where Kahneman was reserved, Tversky was charismatic. It could have gone either way, but when they started talking, something clicked. They began meeting to discuss their hunches about how people make judgments, and those conversations turned into one of the most productive collaborations in the history of psychology.

What they discovered changed everything: humans aren't rational. What we are is predictably and consistently irrational, in very specific ways. Our brains rely on mental shortcuts (called heuristics) to make quick decisions that often lead us to make the same kinds of errors over and over. For instance, if you ask people to judge how common something is they'll base it on how easily examples come to mind rather than actual statistics. These patterns are so reliable you can design experiments around them.

As Amos Tversky’s widow Barbara Tversky (who would later become Daniel Kahneman’s romantic partner—more on this juicy goss in a later piece), explained about their research: “People tend to see patterns and make connections that are not really there and to base decisions on that.”

Kahneman and Tversky ran experiments showing that even trained psychologists and statisticians made these mistakes when reasoning under uncertainty. We anchor our estimates to irrelevant numbers. We judge probability based on how easily examples come to mind. We see our small sample sizes as representative of larger patterns. They called these systematic mistakes cognitive biases and spent the 1970s cataloging them—anchoring bias, availability heuristic, representativeness heuristic, loss aversion.

Their 1974 paper in Science, "Judgment Under Uncertainty: Heuristics and Biases," became one of the most cited works in psychology. They'd essentially founded behavioral economics—a field that challenged the core assumption underscoring economics (that people act rationally) and reshaped how we understand decision-making in everything from medicine to finance to public policy.

The book laid out his theory of two systems of thinking: System 1, fast and intuitive, and System 2, slow and deliberate. System 1 is where most of our cognitive biases live—quick, automatic, and frequently wrong.

By then, the research he and Tversky had started had spawned hundreds of identified cognitive biases.

Tversky died of cancer in 1996, at just 59. Six years later, Kahneman won the Nobel Prize in Economics. (Nobel Prizes aren't awarded posthumously, but Kahneman always insisted it was a joint prize.) In 2011, Kahneman published Thinking, Fast and Slow, which became an international bestseller and introduced their research to millions of readers.

But here's the problem:

Knowing that 200 biases exist doesn't help you spot them in real time. The list was overwhelming, academic, impossible to apply to your actual life.

The research had changed how experts understood the human mind. But it would take someone else—someone outside academia entirely—to make it actually useful.