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There is an appealing simplicity to saying that 85% of a portfolio will be invested for the long term while 15% will be reserved for individual ideas.

The difficulty begins approximately five minutes later.

What happens when one of the individual stocks doubles? What happens when several of them fall? Does the investor restore the original percentages in both cases? Can new money refill the speculative account? When does a successful position stop being an experiment? And what prevents a temporary 15% allowance from slowly becoming half the portfolio?

These questions matter because an allocation is not yet a system. An allocation describes how a portfolio looks on one day. A system describes what happens next.

In A Budget for Speculation: Can Stock Picking and Passive Investing Coexist?, I argued that some investors may be better served by giving speculation a limited budget than by pretending they will never feel tempted to speculate. Most of the portfolio performs the sober work of long-term wealth building. A smaller portion provides room for individual ideas, intellectual curiosity and the occasional attempt to be right in a more interesting way.

The behavioral premise was simple: the appetite gets a plate, not the pantry.

This companion article is about building the plate.

First, distinguish the general strategy from the ratchet

Core–satellite investing is a broad family of portfolio designs. The core generally consists of diversified, long-term holdings. The satellites are smaller positions used to pursue active ideas, specialized exposures or other objectives not fully represented in the core.

There is a wonderfully boring argument for how to build wealth in the stock market.

Buy a diversified collection of productive businesses. Keep buying. Reinvest the dividends. Avoid panicking. Wait a very long time.

For most individual investors, broad-market index funds make this remarkably easy.

There is only one problem.

Stock picking is fun.

Not for everybody, certainly. But for a particular kind of investor, researching individual companies scratches an itch that an index fund simply doesn't. There is a puzzle to solve. A judgment to make. A possibility that everyone else has missed something.

Maybe a company is temporarily unpopular. Maybe a promising business is buried beneath ugly quarterly numbers. Maybe the market has underestimated some technological change. Maybe you have found the proverbial $10 bill lying on the sidewalk.

And, hovering somewhere behind all that sober analysis, there is another possibility:

What if this one really takes off?

This creates a peculiar conflict.

The investor may intellectually believe that diversified, long-term investing is the more dependable path to wealth while simultaneously finding individual stocks much more interesting.

The usual solution is straightforward: suppress the second impulse.

But perhaps there is another possibility.

Instead of trying to eliminate the desire to speculate, give it a budget.

Two jobs that investing can accidentally acquire

For years, one of the most common pieces of investing advice has been remarkably simple: buy a broad market index fund and hold it for the long run.

There is a great deal of evidence supporting that advice. Index funds offer low costs, broad diversification, and a way to participate in the growth of the economy without attempting to outsmart the market. For many investors, they remain an excellent choice.

Still, there is a subtle detail that is easy to miss.

Many investors think of a fund tracking the S&P 500 as a basket containing five hundred companies of roughly equal importance. In reality, that is not how most major indexes work.

The largest companies receive the largest weights.

This means that although an index fund may hold hundreds of businesses, its performance can be influenced disproportionately by a much smaller group.

That distinction may not matter much during ordinary times. It becomes more interesting when a particular sector captures the imagination of investors.

In the current market, artificial intelligence has become one of the dominant narratives. Enormous amounts of capital have flowed toward companies seen as leaders or beneficiaries of the AI boom. Some of these firms have reached valuations that would have seemed extraordinary only a decade ago.

Whether this enthusiasm proves fully justified is a question for the future. The technology itself is undeniably impressive. The economic consequences may be equally profound.

A funny thing happened to cybersecurity company Zscaler over the past week.

The company reported earnings that beat analyst expectations. Revenue came in above forecasts. The business itself did not suddenly stop functioning. Customers did not disappear overnight. The company's products did not become obsolete.

And yet the stock promptly lost roughly a third of its value. A few days later, it changed its mind. The stock rebounded dramatically, recovering much of the decline and ending up surprisingly close to where it had started.

Looking at the chart, someone who had been away from the news might assume some major event had occurred. A scandal. A data breach. A failed acquisition. A lawsuit. Instead, much of the drama revolved around guidance.

Management's outlook for future revenue came in slightly below analyst expectations. Not catastrophically below. Not "the business is broken" below. Just below. Billions of dollars in market value evaporated anyway. Then, with relatively little new information entering the picture, billions of dollars came back.

Zscaler’s daily closing price shows the sharp post-earnings selloff and the rebound that followed.

That sequence has been stuck in my head all week.

The Strange Thing Wasn't the Drop

A stock falling after earnings is not unusual. Investors are not buying the past quarter. They are buying expectations about future quarters.