Markets are rational. Investors are not.

In 1841, the Scottish journalist Charles Mackay published Extraordinary Popular Delusions and the Madness of Crowds, a catalogue of speculative manias, panics, and collective irrationality stretching back centuries. He walked through the tulip mania, the South Sea Company, and the Mississippi Scheme, and the same shape turns up in all of them. Only the details change.

Markets move in cycles of greed and fear, and the same cognitive errors repeat across every generation of investors. We buy after prices have already risen, because rising prices feel like evidence of wisdom. We sell after prices have already fallen, because falling prices feel like evidence of danger. We do exactly the wrong thing, at exactly the wrong time, for entirely understandable psychological reasons.

Daniel Kahneman's research established that the pain of a financial loss is roughly twice as powerful as the pleasure of an equivalent gain. This asymmetry is wiring, not a character flaw. But in a portfolio context, it is lethal. It produces a pattern decades of fund-flow data has documented: the average equity investor consistently earns significantly less than the funds they invest in, because they move money in and out at the worst possible moments.

There is a second illusion working against you, and it belongs to the professionals. When Kahneman studied eight years of results at a wealth-management firm, he went in expecting to find that the better advisers stayed better. Instead the year-to-year correlation in their rankings came out almost exactly zero, which is another way of saying that a good year told you next to nothing about the year that followed. These were serious people, paid handsomely, for what the numbers showed to be a coin flip wearing the costume of talent. Kahneman called it the illusion of skill, and it is the quiet engine underneath nearly every polished track record you will ever be sold.

A rules-based system does something more valuable than making you a better analyst: it removes you from the decision at the exact moments when being human is most dangerous. The rule has no fear and no memory of last week's loss, and it pays no attention to the news. It checks one number against another and reports what it finds.