Diversification and position sizing: why one stock shouldn’t be 40% of your portfolio

What one bad result does to a portfolio

Every company can disappoint: a failed product, a lawsuit, an accounting problem, a competitor that gets ahead. When a stock is a small part of your portfolio, a disappointment like that hurts but doesn’t decide your year. When it is a large part, it can.

Compare a portfolio of 20 holdings of 5% each. Even if one company went to zero, the portfolio would lose 5%. Diversification means spreading your money so that no single result can do outsized damage. It doesn’t make losses impossible; it makes them less dependent on any one thing going wrong.

Two kinds of risk

It helps to split risk into two parts:

• Company-specific risk: things that hit one business, such as a recall, a lost contract or a failed drug trial. Owning many different companies reduces this risk, because one company’s bad news is diluted by everyone else’s.
• Market risk: things that hit almost everything at once, such as recessions, interest rate changes or a financial crisis. Owning more stocks doesn’t remove this risk. In a broad sell-off, a well-diversified stock portfolio still falls.

That second point is often misunderstood. Diversification smooths out the surprises from individual companies; it doesn’t protect you from the market as a whole. Investors who want less market risk usually look at how much they hold in stocks at all, compared with bonds or cash, which is a separate decision.

Correlation: owning many things that move together

Correlation measures how closely two investments move together, on a scale from −1 to +1. Near +1, they tend to rise and fall together. Near 0, their moves are mostly unrelated. Below 0, one tends to rise when the other falls. Diversification works best when holdings aren’t highly correlated.

Owning ten stocks sounds diversified, but if all ten are in the same industry, they often react to the same news: a change in regulation, a shift in demand, a rise in costs. They can fall together. Common sources of hidden concentration include:

• One sector or theme: several companies that depend on the same customers, technology or commodity price.
• Overlapping funds: a broad index fund and a large-company fund often hold the same biggest names, so owning both concentrates rather than spreads. ETFs vs mutual funds vs index funds covers how to check overlap.
• The same stock in several accounts, or held directly and also as a top holding of a fund you own.
• Your employer’s stock: your job and your savings then depend on the same company.

Correlations also change. In a sharp market fall, assets that usually move independently can drop together, so diversification tends to help less in a crisis than it does on average.

How concentration creeps in

Many concentrated portfolios didn’t start that way. A stock that does very well grows to dominate the portfolio on its own.

That is a nice problem to have, but it changes the risk. Rebalancing means periodically trimming holdings that have grown beyond the share you intended and adding to those that have shrunk. Some investors rebalance on a calendar, such as once a year; others when a holding drifts a set amount away from its target. In a taxable account, selling a winner can mean paying tax on the gain, which is one reason people rebalance with new money where they can.

Position-sizing rules of thumb

Position sizing is deciding how much to put into any one holding. There is no correct answer for everyone, but a few common approaches show how investors think about it. These are illustrations, not recommendations:

• A cap per holding. Some investors set a maximum share for any single stock, such as 5% or 10% of the portfolio, and trim when a holding grows past it.
• Equal weights. Splitting money evenly across holdings keeps any one from dominating, and makes drift easy to spot.
• Core and satellite. A large “core” in broad funds, with a smaller portion in individual stocks, so a single company can only affect the satellite part.
• Size by risk, not by conviction. Some traders decide how much they are willing to lose on one position, as a share of the portfolio, and size the position from that.

Seeing your own concentration

It is hard to judge concentration when your money is spread across several accounts. Wenlo’s portfolio combines imported accounts in one view. Its Allocation card breaks holdings down by type, sector, account and top holdings, and a note appears when any single holding is more than a quarter of the accounts you are looking at. That note isn’t a warning to sell; sometimes a large position is deliberate. It is a prompt to check that the size matches what you intend.

Key takeaways

1. Concentration magnifies single results. A 50% drop in a 40% holding costs the portfolio 20%.
2. Diversification reduces company-specific risk, not market risk.
3. Count correlations, not tickers. Ten stocks in one industry can behave like one bet.
4. Winners drift upward in weight. Rebalancing keeps sizes close to what you intended.
5. Decide sizes in advance, with a cap, equal weights or a planned loss, and remember that gaps can exceed the plan.

How much concentration is right depends on your goals, your time horizon and how much loss you could live with. Knowing your actual exposure is the starting point for that decision, and a licensed professional can help you weigh it against your own situation.