Investing 101: How to Build a Diversified Portfolio

A farmer plants four different crops across his land. One year a dry summer ruins the corn, but the beans, squash and wheat do fine, and he gets through. His neighbor planted only corn because corn paid best the year before. The dry summer wipes the neighbor out.

Neither farmer knew which crop the weather would punish. The first one did not have to know. That is diversification, and it is the closest thing investing offers to a free lunch: a way to lower your risk without giving up much in return.

In Parts 1 through 3 you gathered the pieces. Now we assemble them into a portfolio, which is just a word for the whole collection of investments you own.

Spread Your Money Out

The single most important habit in building a portfolio is not putting too much in any one place. Own one company’s stock and your fortune rides on that one company. Own a broad index fund and you own hundreds or thousands of companies at once, so no single failure can sink you. A whole economy does not vanish the way one business can.

You spread out in more than one direction. You spread across many companies, which a broad fund does for you. You spread across the United States and the rest of the world, since other countries do not always rise and fall on the same schedule. And you spread across stocks and bonds, the growth engine and the ballast. A handful of low-cost funds covers all three.

Decide Your Mix of Stocks and Bonds

The biggest decision you will make is how much to hold in stocks versus bonds. This is called your asset allocation, and it depends mostly on one thing: how long until you need the money.

If you are young and will not touch this money for decades, you can lean heavily toward stocks, because you have time to ride out the drops. A common starting point for someone in their twenties might be something like 80 or 90 percent stocks and the rest in bonds. Someone a few years from retirement would hold far more in bonds, because protecting what they have built now matters more than squeezing out the last bit of growth. There is no single right answer, only the answer that fits your timeline and how much bumpiness you can stand without panicking.

A simple, well-respected version of an entire portfolio can be just three funds: a total U.S. stock fund, a total international stock fund, and a total bond fund. That is enough to be genuinely diversified. You do not need anything complicated to start, and complicated is not better.

Keep It Steady With Two Habits

Two plain habits do most of the work of keeping a portfolio healthy.

The first is investing the same amount on a regular schedule, no matter what the market is doing that week. This is called dollar-cost averaging. When prices are high, your fixed amount buys fewer shares; when prices are low, it buys more. You stop trying to guess the perfect moment, which almost nobody can do anyway, and you let the habit carry you. Automating it so the money moves on its own is even better.

The second is rebalancing. Over time, your winners grow and drift your mix away from your target. Say you aimed for 80 percent stocks and a strong run pushes you to 90. Rebalancing means trimming back to your plan and topping up what lagged. If you don’t rebalance your portfolio, the market will likely do it for you, and you may not like the results. Doing it on a set schedule, by policy rather than by mood, keeps your risk where you intended it.

Aim for the Right Goal

It helps to be clear about what you are even trying to do. Investing is not about maximizing your return; it is about earning a competitive return, one sufficient to reach your long-term goals, while managing risk. A portfolio that swings for the fences can strike out at the worst possible moment. A diversified one is built to get you where you are going without forcing you to bet the farm.

Before You Move On

If you remember nothing else about building a portfolio:

• Spreading your money out lowers your risk without costing you much, which is rare in life.
• Your stock-and-bond mix should follow your time horizon, with more stocks when you are young.
• A few broad, low-cost funds can be a complete, diversified portfolio on their own.
• Invest on a schedule and rebalance by plan, not by emotion.

In Part 5, the last piece, we turn the whole thing around and look at how investors trip themselves up, because avoiding a few big mistakes does more for most people than any clever move ever will.