Asset allocation is the strategic distribution of investments across different asset classes — stocks, bonds, cash, real estate, commodities. Research consistently shows that asset allocation explains roughly 90% of the variation in a portfolio's returns over time, far more than individual stock selection or market timing. In other words, the big decision isn't which stock to buy. It's how much of your money lives in stocks at all.
Your allocation should reflect your time horizon, risk tolerance, and financial goals. Younger investors typically hold more stocks for growth potential, accepting bigger swings because they have decades to recover. Those closer to retirement usually shift toward bonds and cash for stability, because a deep loss right before you need the money is the one you can't wait out.
Correlation measures how two assets move in relation to each other, on a scale from -1 to 1. Perfectly correlated assets (correlation of 1) move in lockstep; negatively correlated assets move in opposite directions. Holding assets with low or negative correlations reduces overall portfolio volatility — the bumps partially cancel out.
The math is friendlier than you'd guess. Take two assets that each swing about 15% a year. If they're perfectly correlated, a 50/50 mix still swings 15%. If they're uncorrelated, the mix swings only about 10.6% — a smoother ride with no return given up, just from combining things that don't move together.
Modern Portfolio Theory (MPT), developed by Harry Markowitz, formalized this: for any level of expected return, there's a combination of assets that minimizes risk, tracing out what's called the efficient frontier. MPT changed how professional investors think about building portfolios — the portfolio, not the individual pick, became the unit of design.
Position sizing answers a deceptively hard question: once you've picked an investment, how much should you buy? Size too small and a great idea barely moves the needle. Size too big and one mistake can sink the whole portfolio.
The simplest approach is equal weight: 20 positions at 5% each. It's humble in a useful way — it admits you don't know which pick will be the star.
A more deliberate approach caps the loss per position. Say you run a $50,000 portfolio and decide no single idea may cost you more than 1%, or $500. You want a stock at $40 and you'd give up on the thesis at $36 — a $4 risk per share. $500 divided by $4 is 125 shares, a $5,000 position. Notice what happened: the size came from the risk, not from conviction or excitement.
Whatever method you use, the principle is the same. Decide what a position is allowed to do to your portfolio before you buy it, not after.
Left alone, a portfolio drifts. Your winners grow into a bigger share of the pie, which means yesterday's rally quietly raises today's risk. Rebalancing is the discipline of trimming back to your target mix.
Walk through it. You start with $10,000 split 60/40: $6,000 in stocks, $4,000 in bonds. Stocks jump 30% while bonds stay flat. Now you hold $7,800 in stocks and $4,000 in bonds — $11,800 total, and stocks are 66% of it. You never chose to run a 66/34 portfolio; the market chose it for you. Rebalancing sells $720 of stocks and buys bonds, restoring 60/40.
Notice what the mechanism does: it systematically sells what's gone up and buys what's lagged. That's emotionally hard — you're trimming the thing that's been winning — which is exactly why making it a rule helps. Common approaches are calendar-based (say, once or twice a year) or threshold-based (rebalance when an asset drifts 5 percentage points from target). Both work; the point is having one, and minding taxes and trading costs when you do it.
Diversification's benefits arrive fast, then fade. Going from 1 stock to 10 removes a huge share of single-company risk. Going from 10 to 30 helps meaningfully. Beyond roughly 30 well-chosen stocks across different industries, each new position removes only a sliver of remaining risk — and no number of stocks removes market risk, the tide that lifts and drops everything together.
So there's a real tradeoff. Concentration gives your best ideas room to matter, but a single blowup hits hard. Broad diversification softens any one failure, but your hundredth-best idea drags on your first. Owning two stocks in the same industry also diversifies less than it appears — an airline plus an oil producer spreads risk better than two airlines.
There's no universally right number. There is a right question: if your largest position went to zero tomorrow, would the portfolio — and you — recover?
Portfolio construction is a stack of decisions, made in order. First, allocation: how much in stocks, bonds, and cash, given your horizon and your stomach. Second, diversification: spreading the stock sleeve across enough uncorrelated holdings that no single failure is fatal. Third, sizing: deciding what each position is allowed to cost you. Fourth, rebalancing: a standing rule for pulling the mix back to target.
None of these steps requires predicting the future, which is precisely their charm. They're about controlling what you can control — exposure, concentration, and discipline — so that the market's surprises, which are guaranteed, don't become portfolio-ending events. A well-built portfolio won't dodge every storm. It's built so you can stay invested through them, and staying invested is where long-term returns actually come from.
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