Sometimes I feel like a broken record when I talk about the benefits of index funds. And to be honest, index fund advocates—myself included—are sometimes guilty of being a little preachy. So I won’t bore you with the same facts I’ve cited before.

But the stock market’s behavior this year is worth noting because it illustrates another, more subtle reason to choose index funds—one I haven’t discussed before.

You’ve probably heard the expression, “a rising tide lifts all boats.” When it comes to the stock market, this is generally true—but only sort of and only some of the time.

The reality is that the “stock market” consists of many different industries—everything from technology to healthcare to energy and industrial companies. In all, the S&P 500 breaks down into eleven different industries. And to continue with the rising-tide analogy, each of these eleven industries is like its own small boat. Sometimes they move together, but oftentimes not.

As an individual investor, these dynamics are worth understanding. The past twelve months provide a perfect laboratory for seeing them in action. Between October 3, 2018 and October 3, 2019, the overall S&P 500 was virtually unchanged—up just 1.5%. But pull back the curtain to examine each of those eleven industries, and the results have hardly been uniform. In fact, they could hardly have been more different.

What was the best performing industry over the past twelve months? Probably the last one you might guess: the utilities industry, comprised of electric, gas and water companies. As a group, utilities gained more than 25%. And what was the worst? The energy industry, which is down 24%. In other words, a nearly 50 percentage point difference between the best and the worst. And the communications sector, home of Google, Netflix and other popular stocks—how did it fare? Barely positive, with a return of just 0.5%.

These types of divergences occur all the time, but sometimes they are greater than others. According to an analysis by Goldman Sachs, stocks moved as a relatively tightly correlated group for many years after the 2008 recession, as the market marched higher. But over the past year, that correlation has broken down. In late September, as the market approached record highs, only 106 individual stocks within the S&P 500 were themselves at record highs.

What explains these divergences? It depends. Each industry is driven by its own dynamics. Energy companies, for example, often move in virtual lockstep with oil prices. Other industries will move together in response to a big multi-year trend. The growth of smartphones, for example, has lifted the stocks of all companies in that food chain, from manufacturers to component suppliers to owners of cell towers.

Sometimes groups of stocks will respond to a very specific event. Back in 2011, for example, the stocks of all computer hard drive makers were hit when monsoons swept through Thailand, damaging factories where a large portion of the world’s hard drives are made.

Sometimes stock movements have more indirect causes. During the last three months of 2018, for example, when people began to fret about the impact of higher interest rates, the stocks of high-flying companies took a steep dive. Netflix, for example, dropped 28% in just three months. And where did investors go during that period? Into so-called safe haven stocks like Procter & Gamble, which makes toothpaste, diapers and paper towels—items that people continue buying through good times and bad. That company and its peers tend to do extraordinarily well when people get scared.

Innumerable other factors impact the fortunes of individual industries. These include inflation, regulation and, most recently, tariffs.

To be sure, there are lots of people on Wall Street who make their living tracking things like oil prices, industrial production and a thousand other economic indicators. But in the end, as the last three months have shown, it’s impossible to know where any of these numbers is heading. That’s because so much of what happens in the economy is random, like the flooding in Thailand, or it’s dependent on decisions by policymakers, who themselves probably couldn’t tell you how things will turn out next month or next year.

All of this, in my view, is yet another reason to cast your lot with index funds—and not just any index fund, but broad, total-market funds that will help protect you when a storm hits any one industry.