I’ve just finished reading a weekend piece in the FT this weekend entitled “Stock market casino: the rising stakes of gamified investing,” and it crystallised a thought that I have been mulling over for a while.

When I started out in financial markets in the 1990s, investing was still dominated by the mathematical approach Benjamin Graham made famous: calculate what a company is worth, and expect the share price to converge on that value in time. Then behavioural finance style investing, first academically documented in the 1970s, began to gain real traction this century. But even then, prices remained tethered to some fundamental value you could estimate. After all, economics has always wanted to be physics, not psychology.

And of course human nature hasn’t changed; those opposing emotions of fear and greed still drive markets. What has changed are the tools now available to investors. Forty percent of Gen Z say sports betting is a form of investing, and 26% call it a deliberate part of their long-term financial strategy. Almost half a trillion dollars has traded on prediction markets so far this year, and Kalshi is now valued at more than $40 billion, roughly double the world’s largest sports betting company. New tools change where money goes. More importantly, they change what people see, and that is as true for professionals as it is for twenty-something punters.

Everyone knows the line: if all you have is a hammer, everything looks like a nail. The philosopher Abraham Kaplan put it more vividly: give a small boy a hammer and he will find that everything he meets needs pounding. Charlie Munger later called it “man with a hammer syndrome” and spent decades warning against it.

But the original insight is subtler than the cliché. Our tools don’t just help us solve problems; they decide which problems we notice in the first place. A surgeon only sees a case for surgery, a lawyer sees a liability, an economist sees an incentive problem. We are often just blind to problems that we cannot fix.

After thirty years of managing money, most of them running a hedge fund, I believe this is one of the most expensive biases in our industry. It rarely shows up as an obvious error. It shows up as a quiet narrowing of what we can see, and markets are very good at punishing whatever we miss.

Every style is a hammer. Every investment discipline is a toolkit, and every toolkit comes with its own nails.

The value investor’s discounted cash flow model and love of low multiples make every falling share price look like an opportunity. Sometimes it is. Sometimes it is a business being disrupted, and the cheap stock becomes a value trap that gets cheaper for a decade. The growth investor carries the opposite hammer: total addressable market and revenue momentum make every high-multiple company look like the next platform winner. That worked beautifully until interest rates rose in 2022 and long-duration assets were repriced almost overnight.

The macro manager sees central banks behind every move, even when the real story is one company’s balance sheet. The quant sees a factor exposure and may miss that the factor has become crowded, as many did in the quant unwind of August 2007. The short seller, trained to find fraud, can come to see deception in every set of accounts, including those of genuinely good companies.

None of these tools are wrong. Each is a lens that brings some features into focus and blurs others. The danger is forgetting we are looking through a lens at all.

Why hedge funds swing the hammer harder. In a long-only fund, a narrow lens usually costs some relative performance. In a hedge fund, leverage and shorting turn the same blind spot into an existential risk. Long-Term Capital Management (LTCM) is the classic case. Its partners had some of the finest quantitative tools ever built, and those tools said spreads between related bonds would converge. But the problem in 1998 was not mispricing; it was liquidity and contagion, which their models treated as noise. The hammer was brilliant; the nail was imaginary.

Incentives deepen the groove. A manager who raised capital on a stated strategy is paid to keep using that tool, because investors bought the hammer and style drift is punished. Career risk adds to it: being wrong with the crowd is forgiven; being wrong alone is not. Over time the tool becomes part of our professional identity, and that is exactly when it is hardest to put down.

The answer is not to abandon your edge, since a specialist’s tool is often the source of their returns. The answer is to know where your tool ends and keep others within reach. Munger called this a “latticework of mental models” drawn from psychology, biology, physics and history. An investor who also thinks about incentives, feedback loops and base rates sees a richer problem than one who only sees a multiple.

A few habits that I have learned that have served me well:

• Invert. Before buying, ask what would have to be true for this to be a terrible investment, and whether my tools could even detect it.
• Run a pre-mortem. Imagine the position is down 40% a year from now and write the story of why. The stories you struggle to write point to your blind spots.
• Borrow a rival’s lens. Ask how a macro manager, credit analyst or short seller would describe the same stock.
• Build a team that disagrees. A room full of people with the same training is one hammer swung many times.
• Separate the tool from the self. Investors with a specific methodology should describe a process, not an identity to defend.

What thirty years teaches

Markets are the most reliable humbling machine ever invented, because they rotate which tools work. No tool works at all times. Value struggled through much of the 2010s, then came roaring back. Trend-following looks foolish for years, then saves a portfolio in a single bad quarter. The tool that made your reputation in one regime may quietly cost you in the next.

The managers I admire most aren’t the ones with the sharpest hammer. They are the ones who pause before every swing to ask whether this really is a nail. That question costs nothing, and over a career it may be worth more than any model we build.

So the next time a trade looks obvious, ask yourself: is the opportunity in the market, or in the tool I happen to be holding?