Traders work on the floor of the New York Stock Exchange shortly after the market's opening in New York

As Wall Street hit another new record Thursday, it is worth considering what could cause a serious setback in stock market prices around the world. Since I started writing this column in 2012, I have repeatedly argued that the rebound in stock market prices from their nadir in the 2008-09 global financial crisis was turning into a structural bull market that could continue into the next decade.

Asset prices, however, never move in a straight line. It has been more than two years without even a 10 percent correction and five years without a 20 percent setback. This cannot go on.

Sometime in the not-too-distant future, investors are certain to suffer some big and painful losses — even if I am right in expecting equity prices to continue rising in the long term. What kind of event is most likely to end this bull run, or at least interrupt it with a setback of 20 percent or more?

Bull figures are pictured in front of the German share price index DAX board at the German stock exchange in FrankfurtThe obvious answer is a major economic crisis, such as the near-breakup of the eurozone in 2011-12 or a U.S. recession. Another possible trigger would be a substantial increase in interest rates.

All the worst bear markets in living memory — 1973-74, 1980-82, 2000-02 and 2007-09 — occurred after a series of rate hikes by the Federal Reserve, and monetary tightening is the most widely discussed investment risk today. But on closer inspection, neither economic fundamentals nor monetary policy looks like a serious threat, at least in the year ahead.