William Eckhardt trained as a mathematician and logician before becoming Richard Dennis's trading partner and co-architect of the Turtle system. His contribution wasn't just rules — it was diagnosing why untrained humans trade badly in the first place.
The pain: Eckhardt noticed that even smart, numerate people made the same trading mistakes over and over, and it wasn't from lack of intelligence. Something in normal human reasoning — reasoning that works fine in everyday life — actively sabotages trading decisions.
The lesson: Eckhardt described what he called the "trend fallacy" — our brains are built to expect reversal after a run (a coin that's landed heads five times "feels due" for tails), but markets have momentum, and a trend that's run far often keeps running, for reasons unrelated to a coin flip. He argued traders lose money constantly by intuitively fading strong trends, expecting mean reversion where none was statistically likely.
His answer was rigorous statistical testing over gut feel: test an idea across enough data and enough market conditions before trusting it, because your instinct — however confident it feels — is frequently measuring the wrong thing. This is the intellectual seed of what became modern systematic and quantitative trading: don't trust the feeling, trust the tested rule.
Key Takeaway
The instinct that a stretched move "must" reverse is often just your brain pattern-matching from unrelated experience, not reading the actual market. Test the idea against data before trusting the feeling.
Think About It
When was the last time you faded a strong trend purely because it "felt too extended" — with no other evidence? What happened?
Legend Lab — Test the Fallacy
Look back at the last 10 times a stock or index you follow made a strong, extended move. How many times did it reverse within the next few sessions versus keep running? Write the real ratio down before your next "this has to reverse" trade.