WHEN MARKETS GO crazy, financial writers feel compelled to dust off the keyboard and cook up profound insights. But I am writing this at 5 a.m., while still ingesting my first cup of coffee, so I’m setting the bar a little lower. Here are 12 modest observations following yesterday’s 4.1% plunge by the S&P 500:
1.  I don’t know. You don’t know. Nobody knows. The market turmoil of the past six trading days feels like a sea change after 2017’s remarkable calm.

I FEAR I’M GROWING wealthy at my children’s expense. My investing life began in the late 1980s. Yes, there have been stock market bumps since then, notably the 2000-02 and 2007-09 market crashes, and even a minor hiccup over the past week. But if you look at the broad trend, it’s been three decades of rising stock market valuations.
From year-end 1987 to year-end 2017, the S&P 500’s price-earnings multiple climbed from 13.8 to 24.6,

THE STOCK MARKET had a great 2017, gaining more than 20%. But was that kind of gain justified—or should it worry us, especially after the market had already tripled in recent years? I think it’s useful to understand the range of viewpoints, so we’re better prepared for 2018 and beyond. Here are the bull and bear cases:
Bull Case. As measured by the S&P 500 index, the U.S. market gained nearly 22% last year.

WE CAN VIEW INVESTING as an argument between two competing opinions: What we think an investment ought to be worth—and what the market currently says. It’s an argument the market usually wins.
While we can be highly confident what, say, a certificate of deposit or a Treasury note is worth, it’s much harder to put a value on stocks, gold, high-yield junk bonds and other riskier investments (and, I’d argue, all but impossible with bitcoin).

IF YOUR INVESTMENTS climb in value, hold the champagne—until you figure out whether it’s a onetime gain or a repeatable performance.
Suppose your foreign stocks post gains because the dollar weakens. Or your bonds climb because interest rates fall. Or stocks rise because price-earnings ratios head higher. Or corporate earnings increase because profit margins expand. Or stocks jump because the corporate tax rate or the capital-gains tax rate is cut.
Sound familiar? All of these things have either happened over the long haul or helped drive share prices higher this year. 

I HEAR SO MANY compelling investment arguments. That U.S. stocks are destined to generate lackluster returns because valuations are so rich. That there’s no need to own foreign stocks because you get enough international exposure with U.S. multinationals. That interest rates have nowhere to go but up.
And yet U.S. stocks keep clocking gains, U.S. and foreign shares often generate radically different annual results, and interest rates show no signs of heading significantly higher.

OVER THE 50 YEARS through year-end 2016, the per-share profits of the S&P 500 companies rose a cumulative 1,604%, equal to 5.8% a year, while inflation ran at 4.1%. If share prices had climbed in lockstep with corporate earnings, $1,000 invested at year-end 1966 would have been worth some $17,000 at year-end 2016. On top of that price appreciation, investors would also have collected dividends.
But in fact, over this 50-year stretch, investors fared far better.

INVESTMENT contrarians are having a good year—but not a great one. In 2016, U.S. stocks outpaced foreign shares, smaller companies outperformed their bigger brethren and value stocks beat growth stocks. In 2017, all those roles have been reversed, with foreign shares, big-cap stocks and growth companies topping the performance charts.
For those of us who like to see the mighty fall and the downtrodden lifted up, this has been quite satisfying, except for one small issue: Even as the stock market’s leadership has changed,

“IF YOU DON’T MIND, I have a question for you,” wrote a former colleague. “Should folks be getting out of the stock market? This Trump bump seems like such a crazy bubble.”
Lots of folks are asking this question. How to respond? I fall back on three key points.
First, I believe U.S. stocks are expensive, while foreign stocks are cheap. But that doesn’t tell you anything about short-term performance and only a modest amount about long-run results.