Asset allocation: the single most important choice

Research from the 1980s (Brinson, Hood, and Beebower) found that over 90% of the variability in portfolio returns came from asset allocation — the mix between stocks, bonds, and cash — rather than stock-picking or market timing. The exact percentage is debated, but the qualitative point has held up for 40 years: the mix matters more than the picks.

The core question

How much of your portfolio should be in stocks vs. bonds? Rough rule of thumb: for long-term money, stocks should be the bulk. For money you'll need in under 5 years, bonds and cash should be the bulk. In between, a gradient.

Rules of thumb (pick one to start)

• 110 minus your age = percent in stocks. A 30-year-old: 80% stocks, 20% bonds. A 60-year-old: 50/50.
• 120 minus your age: more aggressive, accounts for longer lifespans and lower bond returns. A 30-year-old: 90% stocks.
• Target-date funds do this for you automatically. If you pick a target-date 2060 fund in your 401k, it will glide from aggressive to conservative over decades without you touching it.

Diversification within stocks

Once you know your stock percentage, split it further: US stocks, international developed, and emerging markets. A common split is 60/30/10 or 70/20/10. The exact numbers don't matter much as long as you're not 100% concentrated in one country or sector.

What different allocations actually feel like

Percentages are abstract until you translate them into worst-case dollars. The table below shows roughly how different stock/bond mixes have behaved historically — the long-run return you'd expect, and what a severe bear market like 2008 would do to a $500,000 portfolio. The question to ask isn't 'which return do I want?' (everyone wants the biggest). It's 'which loss can I actually watch happen to my real money without selling?' Your honest answer to the second question is your allocation.

Notice the asymmetry: going from 60/40 to 100/0 adds about 1.6 percentage points of expected annual return, but nearly doubles the depth of your worst drawdowns. For investors with 30-year horizons and strong stomachs, that trade is worth it. For anyone who checked their balance daily in March 2020 with a knot in their chest, it probably isn't — because the real risk isn't the drawdown, it's what the drawdown makes you do.

A worked example

Meet a 35-year-old with $150,000 saved, planning to retire around 65. Using 110-minus-age, she targets 75% stocks, 25% bonds. Within stocks, she goes 65% US, 35% international. Her actual holdings: $73,000 total US stock fund, $39,000 international stock fund, $38,000 total bond fund. Done. When she's 45, she'll shift toward 65/35. The entire strategy fits on an index card, took twenty minutes to set up, and — this is the important part — requires no forecasts about interest rates, elections, AI, or anything else to work. Allocation is the one investing decision that doesn't require predicting the future, only knowing yourself.

Common allocation mistakes

• Aggressive on paper, conservative in practice. Choosing 90/10 and then selling in the first correction produces worse results than an honest 60/40 held forever. Behavior is part of the allocation.
• Letting drift set your risk. An 80/20 chosen in 2015 became roughly 92/8 by 2024 without rebalancing. If you haven't touched your mix in years, the market has been quietly re-deciding your risk level for you.
• Forgetting to count all accounts. Your allocation is the total across your 401(k), IRA, and brokerage combined — not each account individually. Many people hold aggressive funds in one account and stale cash in another, netting out to a mix they never chose.
• Changing allocation in response to headlines. Your target should change when your life changes — new timeline, new goals, retirement approaching — not when the market does. Reacting to markets is timing, wearing allocation's clothes.

How allocation shifts over a lifetime

Your allocation is not a one-time decision; it is a dial you slowly turn as your timeline shortens. A 25-year-old saving for retirement in 2065 can hold 90 to 100% stocks because a crash today is irrelevant to money that will not be spent for 40 years — in fact, it lets their contributions buy shares cheaply. A 55-year-old planning to retire at 65 usually wants something closer to 60 to 70% stocks, because a major crash in the last decade before retirement, combined with withdrawals, can do permanent damage. This is exactly the glide path that target-date funds automate. If you manage your own allocation, a reasonable habit is to shift roughly 1% per year from stocks to bonds starting about 20 years before you need the money — gradual enough to be painless, steady enough to matter.

One last reality check: allocation percentages feel abstract until you translate them into dollars on your own statement. If you have $400,000 saved, the difference between an 80/20 and a 60/40 portfolio in a 2008-style crash is roughly $60,000 of additional temporary loss. Run that translation before the crash, not during it — deciding in advance what a bad year looks like in dollars is the single best predictor of whether you will actually stay the course.

And remember that all of your accounts form one portfolio: your 401k, IRA, and brokerage account should hit the target in aggregate, even if no single account matches it exactly.