A Decade Rhymes

A small slice can't sink you, and once in a while it does a surprising share of the lifting. The floor is sizing. The quarter was history.

In finance, this shape is called positive skew, often described as asymmetric risk/reward: the most a small slice can lose is the slice itself, and the gain has no fixed ceiling. Research also finds that bets shaped like this can be overpriced.⁴ This page is education about that shape, using one decade that already happened. It isn't a case for owning any of it.

Here's a thing nobody says out loud at a cookout: the part of your money you're most nervous about can be the smallest part of it. If 2% of a pile goes to zero, 98% is still sitting there, doing its job. The downside has a ceiling, and the ceiling is the size of the slice.

The upside doesn't have one. Between the last day of 2015 and the last day of 2025, a 2% Bitcoin slice, trimmed back to 2% every Dec 31 (rebalanced once a year), ended up producing 23.7% of the whole portfolio's growth.^{3 5} About a quarter, rebalanced once a year, from a fiftieth of the money.

We're not going to pretend we knew that in 2015. Nobody did. The decade picked the winner. What the decade can teach is the shape of the bet: same years, other mix, and a slice small enough that losing it is a shrug, not a catastrophe.

Figure 4.1. What's Left If It Goes to Zero

This first picture needs no data at all, which is the point. It's true in any decade, good or bad. In ours, a 2% slice that went to zero in 2016 and was never refilled would have left the pile at exactly 98% of where it would have finished without the slice: 2.383 times the starting dollar instead of 2.432.^3M

One honest wrinkle. That clean 98% is for a slice that's lost once and left alone. If you keep topping the slice back up to 2% every year and the thing keeps falling, the most you can lose is 2% per year, not 2% total. In the actual decade, Bitcoin's two worst years cost the whole pile 1.5% (2018) and 1.3% (2022).^3M It hurt. It didn't break anything.

Figure 4.2. Who Did the Lifting

The rule you pick matters more than people expect, so let's name it every time. Trimmed back to 2% every Dec 31 (rebalanced once a year), the Bitcoin slice produced 23.7% of the decade's growth. Never touched, it produced 74.3%, because it grew until it wasn't a small slice anymore.^{3 5M} Those are two different bets that happen to start in the same place. And in the zero case, the slice doesn't just add nothing; it subtracts 1.4% of the growth.

Now the humble part. Swap Bitcoin for the Nasdaq-100, a broad basket of big growth stocks that had a terrific decade of its own, and the 2% slice produced only 4.2% of growth with the yearly trim, 6.0% left alone.^{6 5M} So the quarter didn't come from "risky stuff" in general. It came from one asset whose price rose roughly 204-fold.³

And most of that came from one year. Bitcoin rose 1,324% in 2017.³ Take 2017 out (call it a flat year) and the 2% slice's share drops from 23.7% to 12.7%.^3M Big outcomes tend to arrive in lumps, not in a smooth drip.

This is where the economics underneath gets interesting. Hendrik Bessembinder looked at every US listed stock since 1926 and found that about 4% of companies account for all of the stock market's net wealth creation, while most individual stocks did worse than Treasury bills.⁷ That's a study of stocks, not crypto, so it says nothing about Bitcoin. What it shows is the shape: returns are lopsided, a few big winners do the heavy lifting, and most concentrated bets underperform. A small slice is one way to have a seat at that lopsided table while capping what it can cost.

Figure 4.3. One, Two, Five

The size of the slice is the dial. At 1%, the Bitcoin slice produced 14.0% of the decade's growth; at 2%, 23.7%; at 5%, 41.1%, all trimmed back every Dec 31 (rebalanced once a year).^{3 5M} The floor moves the other way: lose a 5% slice and you're left with 95% of the pile instead of 98%. That's still arithmetic, still true in any decade. Five percent is also where the Same Years tool starts its Bitcoin slider, so this figure is the bridge between the essay and the tool.

None of this says which size is right for anyone. It says the trade is visible before you make it: you can see the floor, even when you can't see the ceiling.

Picture someone who was 30 at the end of 2015, with $70,000 saved and $500 a month going in. Same years, same paycheck, same habits. The only thing that changes between the two lines is the mix. With the 5% Bitcoin slice, the pile finished $79,664 ahead, 28.0% more. And if that slice had gone to zero on the very first day, the rest would still have finished at $280,174, just $4,468 (1.6%) behind never owning it at all. You still have the rest.

What this is not

This is not a prediction that Bitcoin, or anything else, will repeat its last decade, and it isn't a suggestion to own any of it. It's one window, one asset, and the benefit of knowing how the story ended. What carries over to any decade is the arithmetic of the floor. Education, not advice.

Try it: Same Years. Same years, other mix.

Now try it forward: One Percent, Both Ways. Same idea, your assumptions. (Link is local only; page off the hub until Ryan decides.)