Investment Portfolio

The SEC’s investor site defines a portfolio as the combined holdings of stock, bond, commodity, real estate and other investments owned by an individual or an institution. Most household portfolios are built from three asset categories: Stocks for long-term growth, Bonds for steadier income and a cushion in downturns, and cash equivalents such as money market funds and Treasury bills for near-term needs. Some investors add real estate or commodities, each with risks of its own.

Few people buy these one security at a time. A single mutual fund or exchange-traded fund can hold hundreds or thousands of companies, so a portfolio of three or four broad funds can be more diversified than one holding twenty individual stocks.

Money with a different job usually stays outside the investment portfolio. Your home, car and emergency savings count toward your net worth, but you do not plan to sell them to fund retirement withdrawals, so most planners track them separately. A rental property sits in between: it is an investment, but one you cannot rebalance by selling a small slice.

Harry Markowitz shared the 1990 Nobel Memorial Prize in Economic Sciences for his theory of portfolio choice, first published in 1952. Its central lesson: an investment’s risk depends on how it behaves next to everything else you own. A bond fund that looks dull on its own can make the whole portfolio steadier, because stocks, bonds and cash have historically not moved up and down at the same time. That is the logic of Diversification.

For most households, everything else is spread across several accounts: a 401(k) at work, an IRA, perhaps a taxable brokerage account and a spouse’s plan. Checked one account at a time, the mix can look sensible while the combined portfolio is far more aggressive, or more cautious, than intended, so add up your asset allocation across every account.

Treating the accounts as one portfolio also makes asset location possible. Because every account belongs to the same whole, you can hold bonds where their interest is sheltered and stock index funds where gains are taxed lightly, without changing the overall mix.

Three numbers describe a portfolio better than a list of holdings: its mix, its return and its cost. The mix is each asset category’s share of the total. The return, for a period with no money added or withdrawn, is the weighted average of the holdings’ returns, as in the formula below, and it should be a total return that counts dividends and interest, not just price changes.

New savings are not growth. When you add money during the year, the change in your balance overstates performance, as the worked example shows. Statements handle this with a time-weighted return, which measures the investments alone, or a money-weighted return, which also reflects the timing of your own deposits and withdrawals.

Cost is the third number. Multiply each fund’s expense ratio by its share of the portfolio, add the results, then add any advisory fee charged on the account. Finally, compare the return with inflation: what you can spend later depends on the real return, the growth left after rising prices, not the figure on the statement.

There are roughly four ways to assemble and maintain a portfolio, each trading control for convenience.

Do it yourself with a few broad index funds, as in a three-fund portfolio: costs are lowest, and both the work and the temptation to tinker are yours. Buy an all-in-one fund: a balanced fund keeps a fixed mix, while a target-date fund shifts toward bonds as its target year approaches, but neither can see your other accounts. Use a robo-advisor, which builds a model portfolio from an online questionnaire and rebalances it for an advisory fee on top of fund costs. Or hire a human adviser, who can also coordinate taxes, withdrawals and estate plans, usually at a higher price.

Whichever route you take, the portfolio still needs three things in writing: a target mix, a rule for Rebalancing back to it, and a yearly check that the holdings across every account still match.

Every investment portfolio carries market risk, and the familiar safety nets do not remove it. SIPC protects customers of a failed member brokerage up to $500,000, including a $250,000 limit for cash, by restoring missing securities and cash; it does not protect against a decline in their value. FDIC insurance covers bank deposits, not stocks, bonds or mutual funds, even when a bank sells them. The real defenses are spreading money across and within asset categories, keeping enough cash for near-term needs, and holding a stock share you can live with through a crash. These mistakes undo those defenses most often.