🎯 In 30 seconds: Diversification means spreading money across things that do not all move together, so one bad outcome cannot sink you. The key measure is correlation: how closely two prices move together, from -1 to +1. Owning ten stocks that move as one is not diversified. Adding positions cuts risk quickly at first, then hardly at all, and in a market crash correlations tend to rise just when you need them low. Traders manage this by capping the total risk of all open trades, called portfolio heat.
What diversification is
Investor.gov puts it in the familiar way: do not put all your eggs in one basket. If you own one company and it fails, you lose everything. If you own fifty and one fails, you lose about 2%.
Diversification reduces the risk that comes from any single company, called company-specific or unsystematic risk: a product recall, a fraud, a bad earnings report. It cannot remove market risk, also called systematic risk: the chance that most stocks fall together because of a recession, rising interest rates or a panic.
That second kind is why diversification helps a lot but never makes you safe.
Correlation in plain words
Correlation is a number from -1 to +1 that describes how two things move together.
- +1: they always move in the same direction, in proportion. Owning both is like owning more of one.
- 0: no consistent relationship. One tells you nothing about the other.
- -1: they always move in opposite directions. Gains in one offset losses in the other.
Most pairs of stocks have a positive correlation, because they all respond to the same economy. Two companies in the same industry usually have a higher correlation than two in unrelated industries.
A quick hypothetical example. Here are six months of returns for two made-up stocks in the same industry:
| Month | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| Company X | +2% | -1% | +3% | -2% | +1% | +3% |
| Company Y | +1% | -2% | +4% | -1% | +2% | +2% |
They go up and down in the same months. The correlation works out to about 0.87, which is high. Owning both gives you very little protection if their industry has a bad month.
Correlation is calculated from past data and it changes over time. Six months is far too short to rely on. Real estimates use years of data, and even those shift.
How much does adding positions help?
Here is the core math, using round hypothetical numbers. Suppose each stock has a volatility of 30% a year (a measure of how much it typically swings) and you split money equally.
Two stocks, 50/50:
| Correlation +1.0 | 30.0% (no benefit) |
| Correlation +0.5 | 26.0% |
| Correlation 0 | 21.2% |
| Correlation -0.5 | 15.0% |
| Correlation -1.0 | 0% (perfect hedge) |
Many stocks. If every pair has the same correlation, portfolio volatility is:
portfolio volatility = stock volatility x square root of (1/n + (1 - 1/n) x correlation)
where n is the number of equal-sized positions.
| Number of stocks | Correlation 0.3 | Correlation 0.6 |
|---|---|---|
| 1 | 30.0% | 30.0% |
| 5 | 19.9% | 24.7% |
| 10 | 18.2% | 24.0% |
| 20 | 17.4% | 23.6% |
| 50 | 16.8% | 23.4% |
| Unlimited | 16.4% | 23.2% |
Two lessons. First, going from 1 stock to 10 cuts volatility a lot. Going from 20 to 50 barely changes it. Second, correlation sets a floor you cannot diversify below. Stocks that move together leave you with most of the risk no matter how many you own.
Concentration risk and hidden overlap
Concentration risk is having too much riding on one thing. It is obvious when one stock is 40% of your account. It is less obvious when several positions are really the same bet.
- Sector overlap. A chip maker, a chip equipment company and a software company are three tickers, but often one bet on technology. See Sectors and the business cycle.
- Factor overlap. A factor is a shared driver of returns, such as company size, value versus growth, momentum or sensitivity to interest rates. Five fast-growing, unprofitable companies from different industries can still fall together when interest rates jump.
- Fund overlap. An S&P 500 fund plus a large-company technology fund holds many of the same companies twice. Check the top holdings of each fund. See ETFs and index funds.
- Life overlap. Owning a lot of your employer's stock ties your savings and your paycheck to the same company.
Correlations rise in a crash
Diversification works least well exactly when it is needed most. Ang and Chen, in the Journal of Financial Economics (2002), found that correlations between US stocks and the overall market are much higher during market declines, especially extreme ones, than during rises.
In plain words: on calm days your stocks look nicely independent. In a sell-off, many of them drop together. Run the formula above with ten stocks: at correlation 0.3, volatility is 18.2%. If correlation jumps to 0.9 in a panic, it is 28.6%, almost as risky as a single stock.
So plan for crash-time correlation, not calm-time correlation. Keep total risk modest, and remember that cash and high-quality bonds have historically behaved differently from stocks in many, though not all, sell-offs.
Portfolio heat: total open risk
Portfolio heat is the sum of the risk on all open trades: what you would lose if every stop were hit at once. Each trade's risk is shares x (entry - stop), or zero once the stop is at or above your entry.
A hypothetical $10,000 account with five open trades:
| Position | Shares | Entry | Stop | Open risk |
|---|---|---|---|---|
| Chip maker | 33 | $50.00 | $47.00 | $99.00 |
| Chip equipment | 41 | $40.00 | $37.60 | $98.40 |
| Software | 20 | $60.00 | $55.00 | $100.00 |
| Utility | 50 | $25.00 | $23.00 | $100.00 |
| Grocery chain | 125 | $20.00 | $19.20 | $100.00 |
| Total heat | $497.40, about 5% |
The positions use $8,240 of the account, but the total planned risk is about 5%. That is a common ceiling: many traders cap heat somewhere around 5% to 6% and do not open new trades above it.
Add a second rule for correlated groups, for example "no more than 2% total risk in one sector or theme." In the table, the three technology positions carry $297.40, about 3%, so this trader would skip the next tech setup until one of them closes or its stop moves up.
How many positions?
There is no magic number. For long-term investors, a broad index fund holds hundreds of companies in one purchase, which is why many people use one as the core of their money (see A plan for a young person).
For an active trading account, the limit is usually attention, not math. Each position needs a stop, a plan and some monitoring. Many beginners do better with three to six open trades than with fifteen they cannot follow. Use the heat and sector caps to control risk, and the table above to see that a handful of truly different positions gets you a large share of the benefit.
Try it
Write down every position in your paper portfolio (or a pretend list of five stocks). Next to each, write its sector and the open risk in dollars. Add up total heat and the largest sector total. Then use the stock screener to find one candidate from a sector you do not hold yet, and ask whether adding it would actually make your list more diversified or just bigger.
- What kind of risk can diversification not remove? Answer: Market (systematic) risk: the chance that most stocks fall together because of a recession, rising rates or a panic.
- What tends to happen to correlations between stocks in a market crash? Answer: They rise, so diversification works least well exactly when you need it most.
- What is portfolio heat? Answer: The total risk on all open trades: what you would lose if every stop were hit at once.
- Your three technology positions carry about $297 of risk (3%) in a $10,000 account, and your rule is no more than 2% total risk in one sector. A new tech setup appears. What do you do? Answer: Skip it until one of the tech trades closes or its stop moves up.