Here's a fact that's hard to believe today: in 1980, Kmart was bigger than Walmart. By an order of magnitude.
Kmart had about 1,800 stores. Walmart had 276. Kmart did roughly $14 billion in annual sales. Walmart did about $1.2 billion. When Sam Walton was asked about the competitive landscape, he reportedly described Walmart as "a flea fighting an elephant." Walton was the flea. Kmart was the elephant.
By 2002, Kmart filed for Chapter 11 bankruptcy. By 2025, Kmart has roughly three stores left in the entire United States.
What happened to Kmart between 1980 and 2002 is one of the most studied corporate collapses in business school history. The conventional explanation is "Walmart out-executed them on supply chain and tech." That's part of it. But the deeper explanation is behavioral. Kmart fell into one of the most reliable failure patterns documented in cognitive psychology.
It's called Overconfidence Bias, and it's the single most likely way a market leader dies.
How Don Moore Mapped Three Kinds of Overconfidence
Don Moore, a behavioral scientist at UC Berkeley, has spent his career documenting overconfidence. In his 2020 book Perfectly Confident, he breaks the phenomenon into three distinct flavors:
Overestimation — believing your actual performance is better than it is. (The CEO who thinks their company is more efficient than the market data shows.)
Overplacement — believing you're better than other competitors when you're not. (The Kmart exec in 1985 who thinks Walmart is a regional player that will never matter nationally.)
Overprecision — being too certain about your forecasts. (The Kmart board that confidently projected continued growth into the 1990s without modeling competitor threat.)
Kmart suffered from all three at once. The internal Kmart documents that came out during the 2002 bankruptcy proceedings — and are reproduced in part in Charles Fishman's The Walmart Effect — show a leadership team that fundamentally did not believe Walmart could threaten them, right up until Walmart had quietly surpassed them in store count and revenue.
The behavioral mechanism is straightforward once you see it. Past success creates evidence for the brain that current methods are correct. When competitors do something different, the overconfident brain interprets that difference as evidence that the competitors are wrong, not as evidence that the world has changed.
Walmart, throughout the 1980s, was doing things Kmart explicitly mocked. Walmart was an early adopter of barcodes. Kmart wasn't. Walmart launched a private satellite network in 1987 — the largest privately-owned satellite network in the world at the time — to coordinate inventory and pricing across stores. Kmart didn't have one. Walmart shared point-of-sale data with suppliers in near-real-time so manufacturers could restock proactively. Kmart's data systems lagged by weeks.
Each of these moves was, individually, a small operational improvement. Stacked over fifteen years, they produced an insurmountable cost advantage. Walmart could price 5–10% below Kmart on every comparable SKU and still earn a higher margin.
The Innovator's Dilemma in Two Storefronts
If you've read Clayton Christensen's The Innovator's Dilemma, you'll recognize this pattern. Christensen documented dozens of industries where market leaders failed to respond to a lower-margin, technologically-inferior new entrant, only to watch the new entrant climb up the value chain and destroy them. His case studies were mostly in tech (disk drives, mini-computers, steel). But Kmart-vs-Walmart is the same pattern in fluorescent-lit retail.
Christensen's argument was that market leaders are trapped by their own success. The processes and customer relationships that produced their dominance also produce a powerful inertia that prevents them from responding to threats from below. To respond, a market leader has to cannibalize its own profitable business — which feels insane to a profitable business.
This is what Jim Collins, in How the Mighty Fall, calls "the disciplined pursuit of more." Kmart kept doing what had worked. Walmart kept doing what was going to work. The gap between those two strategies, compounding annually, swallowed Kmart whole.
The Cognitive Move Underneath All of This
The deeper issue, the one Don Moore keeps returning to in his research, is that overconfidence is largely invisible to the person experiencing it. Confidence and competence feel the same from the inside. The Kmart executives weren't being arrogant in a way that they could have noticed and corrected. They were being normal — which is the dangerous part.
The defenses against overconfidence are mostly external. You need outside calibration — competitors who genuinely worry you, advisors who genuinely disagree with you, data that genuinely surprises you. Without those, your internal compass will keep telling you that your current strategy is correct, even as the ground shifts under it.
If you want the operator's reading list on this, three books: Christensen's Innovator's Dilemma for the structural argument, Collins's How the Mighty Fall for the case studies, and Annie Duke's Thinking in Bets for the decision-quality framework. Stack those three and you have most of the toolkit for noticing overconfidence before it eats you.
What I Take From This
The Kmart story isn't really about Kmart. It's about every market leader. Every dominant business that's currently confident in its position is at risk of becoming the next Kmart — and the risk is highest when the confidence is most justified.
The flea always becomes the elephant eventually. The question is just whether the current elephant notices in time to do something about it.