Invest for the long term and disregard day-to-day market movements.

That’s standard advice, and it has merit. For one thing, it frees up precious time for more pleasant pursuits. For another, it encourages the steady buy-and-hold approach that has produced good results for stock investors over the past century.

The conventional wisdom contains a couple of major pitfalls, however.

The first, and most obvious, is that short-term market movements sometimes lead to profound changes. Miss them and you will pay dearly for your inattention. Amid the Trump administration’s abrupt policy shifts — the off-again, on-again threats of new tariffs on Chinese goods that have been wrenching the markets — who can be sure that the vertiginous swings of the past week are minor events?

Shrugging off that short-term volatility isn’t the only challenge. To make matters worse, the conventional wisdom also glosses over an important assumption: Long-term stock market performance is stable and widely understood. At the moment, that appears to be a fallacy.

What is an investor to do when the long view of the market changes, not because of anything happening now, but because of events that took place a decade or more ago?