Building a Portfolio: Asset Allocation and Rebalancing
ℹ️ This is education, not investment advice. It teaches how allocation and rebalancing work and how to write your own — it does not recommend any particular split, fund or product.
The big decision isn't the one everyone argues about
Every finance conversation is about which stock, which coin, which hot pick. That's the small decision. The one that actually explains most of how your money does over the years is boring and almost never discussed: how you split it across asset classes in the first place — the mix of stocks, bonds and cash.
The research behind this is decades old. Studies of pension funds (Brinson and colleagues) found that the allocation policy — the long-run mix — explained the large majority of the variability in returns between portfolios, far more than security selection or market timing. You can win the game everyone plays (picking) and still lose, because you lost the game no one talks about (allocating).
Why nobody explained it this way
Because allocation doesn't sell. "Set a stocks/bonds mix and mostly leave it alone" is a one-sentence strategy — no daily content, no signals, no reason to keep watching. Stock-picking is a story with heroes and drama; allocation is a spreadsheet. So the decision that matters most gets the least airtime, and beginners pour all their attention into the 20% of the outcome and none into the 80%.
1. Asset allocation: the mix is the strategy
Asset allocation is how you divide money across asset classes that behave differently:
- Stocks — highest expected return, highest volatility (MN-05). The growth engine.
- Bonds — lower expected return, steadier; often move differently from stocks, which is what makes them useful ballast.
- Cash — no growth, no volatility; safety and things you'll need soon.
The whole point of mixing them is MN-05's free lunch: because the classes don't move in lockstep, a blend swings less than stocks alone while still capturing much of the growth. A 60% stocks / 40% bonds portfolio isn't a compromise nobody wanted — it's a deliberate dial between growth and steadiness.
2. Setting the dial from your risk profile
You don't pick a mix from a magazine; you derive it from the risk profile you built in MN-05 — horizon, capacity, temperament.
- Longer horizon → more stocks, because time turns volatility into a temporary problem.
- Lower capacity or temperament → more bonds and cash, because you can't afford — financially or emotionally — to sit through the deep drops stocks deliver.
Old rules of thumb exist ("hold your age in bonds," "110 minus your age in stocks") and they're fine as a starting anchor — but they only see age, and your profile is three inputs, not one. The binding constraint from MN-05 still rules: a long horizon doesn't help if your temperament sells at the bottom. Set the dial to the mix you'll actually hold through a crash, because a plan you abandon at the worst moment is worse than a more conservative plan you keep.
3. Diversify within each sleeve, too
Getting the stocks/bonds split right isn't enough if your "stocks" are ten companies in one country and one industry. Within each class, spread across:
- Geographies — your home country isn't the whole world's economy.
- Sectors — so one industry's bad decade doesn't sink the sleeve.
- Company sizes — large, mid, small.
The clean way to get all of that in one holding is a broad index fund (MN-03): a total-market or world index fund is diversification within the sleeve, in a single low-cost line. You're not picking companies; you're buying the whole class.
4. Rebalancing: the discipline that keeps your risk where you set it
This is the part almost everyone forgets, and it's what makes an allocation a living thing rather than a one-time guess.
Set 60/40 and wait. If stocks have a great year, they grow faster than bonds, and your mix drifts — say to 75/25. Nothing did anything wrong, but you are now holding a riskier portfolio than the one you chose, right after a run-up, which is exactly when risk is most likely to bite.
Rebalancing = selling some of what grew and buying what lagged to return to your target. Two things about it:
- It mechanically forces "sell high, buy low." You trim the winners (now expensive) and top up the laggards (now cheaper) — the opposite of the emotional instinct to pile into whatever just went up. You don't need willpower; the rule does it.
- Its job is risk control, not return-chasing. Over the long run rebalancing mostly keeps your volatility where you chose it; it isn't a trick to beat the market, and treating it as one leads to overtrading.
How often: two honest options — calendar (e.g. once a year) or threshold (rebalance when a class drifts more than, say, 5 percentage points from target). Both work; the trap is doing it constantly. Every rebalance can trigger costs (spreads, commissions) and, outside tax-sheltered accounts, taxable gains — so more frequent isn't better. Once or twice a year, or on a threshold, is plenty.
What this means for you
Your portfolio is really two decisions, not a hundred: the allocation (set once from your risk profile, matters most) and the rebalancing rule (maintain it so your risk doesn't drift). Get those two right with broad index funds inside each sleeve, and you've done the part that explains most of the outcome — while the crowd fights over the part that explains the least. None of this is a recommendation of any specific mix; it's the structure you'd fill in with your own numbers.
Try it: write your own allocation (30 min)
- Start from MN-05. Restate your risk profile in one line: horizon, capacity, temperament, and the binding constraint.
- Set a target split across stocks / bonds / cash that you believe you'd hold through a 30% stock drop. Write the three percentages; they must sum to 100.
- Choose a vehicle per sleeve (conceptually — no need to buy anything): name a broad index fund for the stock sleeve (total-market or world), a broad bond fund for bonds, and where cash would sit. Note the kind of fund, not a recommendation.
- Write your rebalancing rule in one sentence: calendar (which month) or threshold (how many percentage points of drift triggers it).
- Stress-test it. Assume stocks fall 30% next year and bonds are flat. Recompute your split — how far did it drift, and would your rule tell you to rebalance? Would you actually do it?
✅ Finish check: you have a written allocation (three percentages summing to 100), a named index vehicle for each sleeve, a one-sentence rebalancing rule, and a drift figure from the stress test.
Summary card
- Allocation — your long-run asset mix — explains most of your outcome, far more than which securities you pick.
- Derive the mix from your risk profile (MN-05); set it to what you'll hold through a crash, not what looks brave.
- Diversify within each sleeve (geography, sector, size) — a broad index fund does this in one line.
- Rebalancing returns a drifted mix to target; it mechanically sells high and buys low and exists for risk control, not extra return.
- Rebalance on a calendar or a threshold, not constantly — each trade has costs and possible taxes.
- Two decisions run the whole thing: the allocation and the rule that maintains it.
Sources
- Brinson, Hood & Beebower — Determinants of Portfolio Performance (1986, 1991)
- Bogle, J. — Common Sense on Mutual Funds
- U.S. Securities and Exchange Commission — Investor.gov: Asset Allocation, Diversification, and Rebalancing
- Markowitz, H. — Portfolio Selection, Journal of Finance (1952)
Next lesson: DT-01 — Reading Data: Mean, Median, Spread (L1) Related: MN-05 Risk, Volatility and Diversification (L2) · MN-03 Index Funds and ETFs (L1) · MN-07 Inflation, Interest Rates and Currency (L2) Path: Company Reader — 4/5