You bought several stocks and they all fell together. Diversification is about how independently holdings move, not how many you own.
If you split your money across five chipmakers, you own five names but have essentially made one bet. Companies in the same industry, the same country and the same economic cycle receive good and bad news together. The benefit of diversification comes not from how many holdings you have but from how differently they move. So before counting positions, ask whether they rise and fall for the same reasons.
The correlation coefficient sums up, in one number, how much two assets moved in the same direction. It ranges from -1 to 1 and is calculated from past returns. It says nothing about cause and effect; it only describes how closely the two moved during the period you measured.
Suppose you put half your money into each of two assets, both with annual volatility of 20%. If their correlation is 1, the combined volatility stays at 20%. At a correlation of 0.5 it drops to about 17.3%, at 0 to about 14.1%, and in theory at -1 it reaches zero. The expected return is simply the average of the two, yet the swings shrink. That is the heart of diversification. Mixing highly correlated assets, however many, produces almost none of this effect. These figures illustrate the arithmetic; they are not values for any real asset.
Events specific to one company, such as weak earnings or a management scandal, tend to cancel out when you hold many companies. Risks that move the entire market, such as interest rate shifts or a recession, remain no matter how many stocks you add. That is why an index fund holding hundreds of stocks still falls when the market falls. Reducing that remaining risk means looking at assets other than stocks or adjusting how much you hold in risky assets at all.
Assets that normally move independently frequently drop together when markets are badly shaken. When fear spreads, people sell risky assets of every kind and reach for cash. If you take comfort in a low correlation measured during calm periods, you may find the benefit has shrunk exactly when you need it. When checking correlations, look at the major drawdown periods separately as well as the full history; that view is closer to reality.
Portfolios that look diversified are concentrated in one direction more often than people expect. You may own several funds that, once opened, hold the same large companies in heavy weights, or split between domestic and foreign stocks only to find both tilted toward technology.
Even if you start with balanced weights, the assets that rise the most gradually take up a larger share. Left alone, the portfolio becomes more lopsided than intended, and the blow is bigger when that asset turns. Many investors therefore rebalance back to target weights on a set schedule or when the gap passes a threshold. Each trade has fees and possibly taxes, and the net benefit depends on how gains are taxed, so weigh the costs too. Rebalancing does not guarantee returns; it is better understood as a way to keep risk at the level you originally chose.
Diversification is not something you set once and forget; it needs regular review. Working through the items below exposes the gap between apparent and real diversification. You can calculate the past correlation between your holdings with a stock comparison tool. This article explains concepts only, does not recommend buying or selling any asset, and is not investment advice.
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