Portfolio Concentration Risk: Too Exposed to One Stock?

Concentration risk, and the base rates that make it dangerous

Concentration risk is simple to define and easy to ignore. It is the share of your wealth that depends on the fate of a single company, sector, or bet. When that name does well, concentration feels like conviction. When it breaks, it is the fastest way to erase years of careful saving.

The base rates are sobering. J.P. Morgan Asset Management, studying every stock in the Russell 3000 from 1980 through 2020, found that roughly 40% of companies suffered a catastrophic loss, a decline of about 70% from peak that was never recovered, and that two-thirds of individual stocks underperformed the index itself. Owning the market rewarded you. Owning any one name was, more often than not, a losing bet against the average.

The thesis of this piece is straightforward. Single-name risk is measurable, it is usually larger than you think once you count every account, and the fix is a sequence of specific trades that most people never get around to making. An agent that can see across all of your accounts changes that, because it measures the exposure continuously and proposes the moves for you to approve.

How to measure concentration risk

Start with the only number that matters: what percentage of your total investable net worth is one position, right now, across every account. Not just your brokerage. Your 401(k), your IRA, your taxable account, your crypto wallet, and any employer equity all count toward the same ticker if they hold the same ticker.

A common rule of thumb treats anything above 10% in a single stock as concentrated and anything above 20% as a serious risk that deserves a plan. These are heuristics, not laws, but they are useful anchors. If one name is 30% of your portfolio, a 70% drawdown in that name, well within the historical range above, takes roughly 21% off your entire net worth in one move.

Two refinements make the number honest. First, look at correlated exposure, not just the exact ticker. If you work at a chip company, own its stock, and also hold a semiconductor ETF, your true bet on that one industry is larger than any single line item suggests. Second, weight by liquidity and lockups. A position you cannot sell for two years carries more risk than the same dollar amount you could exit tomorrow.

Why RSUs and options create hidden concentration

Equity compensation is where concentration hides in plain sight. Restricted stock units, employee stock purchase plans, and vested options are all exposure to one company, and they usually are not in the brokerage account you check each week. They sit in a separate equity platform, so they never show up in the mental math.

The exposure compounds in a way salaried workers escape. Your paycheck, your bonus, your unvested grants, and your existing shares all depend on the same employer. A bad year for the company can hit your income and your portfolio at the same moment, which is exactly when you would least want to sell. That is correlation risk stacked on top of concentration risk.

Options add leverage and a clock. Vested in-the-money options behave like a magnified stock position, and their value can evaporate faster than the shares themselves. The practical failure mode is familiar: people hold concentrated grants for the tax treatment or out of loyalty, watch the position swell to half their net worth during a bull run, and never trim on the way up. The discipline problem is not knowledge. It is that nobody is watching the combined number and prompting action.

The math of single-name risk

Concentration hurts through volatility drag and tail risk, and both are quantifiable. A diversified index might carry annual volatility around 15%. A single large-cap stock often runs 30% to 50%, and smaller names higher. Higher volatility mechanically lowers your compound return over time even when the average return is identical, because losses require larger subsequent gains to recover. Down 50% needs up 100% just to break even.

Tail risk is the sharper edge. Using the Russell 3000 history above, the odds that any given stock is the one that halves and never comes back are not trivial, they are close to a coin flip over a long enough horizon. Diversification does not raise your expected return so much as it cuts the variance of outcomes and removes the small chance of a permanent, portfolio-ending loss.

Here is the intuition in one line. A diversified portfolio can have a bad decade. A concentrated one can have a bad Tuesday. Position sizing is the lever that decides which kind of bad day is even possible for you.

How an agent flags and fixes it across accounts

This is exactly the kind of problem software should own, because it requires seeing everything at once and acting at the right moment. Tengu is AI for investing that connects your banks, brokerages, and crypto so it can measure true exposure across your whole net worth, not one account in isolation. When your employer stock plus your ETF overlap plus your old 401(k) holdings quietly cross a threshold you set, the agent flags it in plain English.

The workflow is find, propose, route. The agent finds the concentration, explains why it matters given your specific holdings, proposes a concrete de-risking plan, and routes the trades to your broker the moment you approve. It is consent-first and non-custodial by design. You keep your accounts and your broker, you approve every move, and you hold a kill switch. It never moves money without you, and where a broker does not support routing, it proposes the trade and you place it yourself.

For those who want ongoing management, you can hire an agent to work inside limits you set, such as a maximum position size, a drawdown ceiling, and a defined universe. The agent trims toward your targets over time rather than in one taxable lump. This is a different animal from legacy robo-advisors, which Fortune in 2026 described as a generic, incremental feature at best. The direction of travel is clear: Deloitte projects that generative AI could become the leading source of retail investment advice in 2027 and reach 78% usage by 2028.

Unwinding a concentrated position without a tax bomb

The reason people stay concentrated is rarely conviction. It is the tax bill on selling appreciated shares. A good plan unwinds the position in a way that respects that, and this is where cross-account intelligence earns its keep.

Sequencing matters. Sell your highest-basis lots first to minimize the gain per share, harvest losses elsewhere in the portfolio to offset the gains you do realize, and spread sales across tax years to stay under bracket thresholds. Tengu's signature capability is cross-account tax-loss harvesting, which pairs the two sides of that trade automatically: it finds a loss in one account to offset the gain you need to take in another.

One rule the agent enforces so you do not have to. The IRS wash-sale rule, Internal Revenue Code Section 1091, disallows a loss if you buy a substantially identical security within 30 days before or after the sale, a 61-day window centered on the trade. Harvest a loss and rebuy the same name too soon and the deduction is deferred, not lost, but the benefit slips away this year. Tracking that window across every one of your accounts by hand is exactly the kind of thing people get wrong, and exactly what an agent that sees all of them gets right.

Run the number, set your ceiling, automate the watch

Run the number this week. Add up every holding of your largest position across all accounts, including RSUs, ESPP shares, and vested options, and divide by your total investable net worth. If it is above 10%, you have a decision to make. Above 20%, you have a plan to write.

Set your ceiling and your floor. Decide the maximum percentage you are willing to hold in any single name and the pace at which you will trim toward it. Write down the tax cost of getting there and the loss-harvesting opportunities that could offset it. Then automate the monitoring, because the failure mode is never the plan, it is forgetting to look. An agent watching the combined figure across accounts, flagging breaches, and proposing the exact trades to fix them turns a good intention into a routine you actually follow.