Asset Allocation

You’ve been investing regularly into a total stock market index fund. You set up dollar cost averaging, automated the purchases, and stopped thinking about it. Good. The next decision isn’t which fund to add. It’s how to split your money across different types of investments. Most people skip right past this decision. It matters more than anything else you’ll do with your money.

What Asset Allocation Actually Is

Asset allocation is the recipe, not the ingredients. It’s how you divide your money between categories of investments. The index funds you buy are the ingredients. Two investors using the same funds but different recipes will have completely different experiences.

There are three main asset classes most investors need to care about: U.S. stocks, international stocks, and bonds. That’s it. You already own the first one. You learned about the other two in Index Funds. The question now is what percentage of your money goes into each bucket.

This one decision explains roughly 88% of the variation in returns between portfolios. That number comes from a Vanguard study that analyzed decades of portfolio data. The fund you agonized over? About 12% of your outcome. The stock/bond split you probably haven’t thought about? The other 88%.

What Different Mixes Look Like

The difference between an all-stock portfolio and a 60/40 split isn’t the return. It’s whether you sell during a crash. Here’s what different mixes have delivered, using U.S. data going back to 1926:

Stock/Bond Split	Average Annual Return	Best Year	Worst Year
100/0 (all stocks)	~10.1%	+54%	-43%
90/10	~9.7%	+49%	-39%
80/20	~9.2%	+45%	-34%
70/30	~8.7%	+41%	-30%
60/40	~8.1%	+36%	-26%

The returns column tells one story. The worst year column tells the only one that matters.

Put it in dollar terms. Say you have $50,000 invested. In a crash year:

Stock/Bond Split	Your $50,000 Becomes	You “Lost”
100/0	$28,500	$21,500
90/10	$30,500	$19,500
80/20	$33,000	$17,000
70/30	$35,000	$15,000
60/40	$37,000	$13,000

With an all-stock portfolio, $21,500 vanishes. With a 60/40 split, $13,000. Both hurt. One is far more likely to make you panic-sell and lock in real losses.

The tradeoff is straightforward. More stocks means higher long-term returns. More bonds means a smoother ride.

Every worst-year drop in that table is temporary. But “temporary” can mean two years or ten years, and the recovery only counts if you’re still invested when it arrives.

How to Pick Your Mix

Time horizon is the single biggest factor. Money you need in two years should not be in stocks. Money you won’t touch for thirty years can afford to ride out multiple crashes.

A common starting point: subtract your age from 110 and put that percentage in stocks. If you’re 30, that’s 80% stocks and 20% bonds. If you’re 50, that’s 60/40. Use that number as your starting point. Then stress-test it.

The real test: imagine stocks drop 40% tomorrow. Your $100,000 portfolio falls to $72,000 in a month. Do you keep investing on schedule? Or do you sell everything and move to cash?

Be honest. If that scenario makes you physically uncomfortable, you need more bonds. If your honest reaction is “I’d buy more,” you can handle more stocks.

Temperament matters more than math here. A 90/10 portfolio earns better returns than 70/30 over decades. But if 90/10 causes you to panic-sell during the next crash, you’d have been far better off with 70/30 and steady hands. The best allocation is the one you actually hold.

Nobody knows their real risk tolerance until they’ve lived through a crash. If you’ve never watched your portfolio drop 30%, start more conservative than you think you need to. You can always add stocks later. Going the other direction, selling stocks after they’ve already fallen, is the most expensive mistake in investing.

The Split Matters More Than the Funds

Most people get the priority backwards. They spend weeks comparing index funds, reading expense ratio differences of 0.01%, and agonizing over total market versus S&P 500. They spend zero minutes thinking about how much of their portfolio should be in bonds.

Remember the 88%? An 80/20 portfolio using mediocre funds behaves almost identically to the same split using the “best” funds.

This is liberating. You don’t need to find the perfect international fund or compare seventeen bond options. Pick reasonable low-cost index funds in each category and get the percentages right. You already made the hard decisions: index funds over active management, automatic investing over guesswork. The specific fund within each asset class is a rounding error.

Stop optimizing the 12%. Get the 88% right.

Where This Breaks

In 2022, stocks dropped 19% and bonds dropped 13%. Both fell at the same time. That’s not supposed to happen. Bonds are supposed to cushion stock market losses, and for most of modern history they have. But not always.

Asset allocation manages risk. It doesn’t eliminate it. Some years, every asset class falls. Some decades, international stocks trail U.S. stocks so badly that you question why you own them at all. (Then a different decade arrives and the relationship flips.) Rules of thumb like “110 minus your age” don’t account for whether you have a stable job, a mortgage, a spouse’s income, or expensive medical needs. Your circumstances are yours.

Over time, your allocation will drift. If stocks rise faster than bonds for a few years, your 80/20 portfolio might become 88/12. You’ll need to rebalance periodically, selling a bit of what’s grown and buying a bit of what’s lagged, to bring the split back to your target. Rebalancing covers exactly how. For now, know that it needs to happen roughly once a year.

If you already own a single fund, you don’t need to sell it. You can shift your allocation gradually by directing new contributions into the asset classes you’re missing. When you’re ready for specific fund picks and exact percentages, The Three-Fund Portfolio turns these principles into a concrete plan.

What’s Next

You know what to own and how to split it. The next thing silently eroding your returns is costs. Expense ratios, trading fees, advisory fees, tax drag. Small percentages that compound into enormous dollar amounts over decades. That’s Fees and Expenses: the money you’re paying that you might not even know about.