Forecasters are prone to focus on a single story line, particularly one as compelling as angry populism. The consensus view figured Mr. Trump’s tax and spending plans would increase growth in the United States, while his protectionist threats would undercut the rest of the world. Most forecasters share the establishment disdain for Mr. Trump, yet few paused to consider whether a leader they see as inept and divisive could deliver all this as fast as he promised.

He didn’t, and the widely hyped “Trump Bump” barely surfaced in 2017. Yes, the United States economy grew around 2 percent and generated jobs at a healthy pace. But both trends date to well before Mr. Trump. Liberals who thought Mr. Trump would induce a global recession proved even farther off the mark.

If one were to avoid the straight-line projections, political biases and single factors that so often distort forecasts, what would a 2018 forecast look like? Not like the consensus, which is euphoric over the current combo of high growth and low inflation. Usually staid Wall Street economists are giving their 2018 forecasts headlines like “Boom Shaka-laka-laka” and “As Good as It Gets.” Inspired by a cocksure consensus, investors are holding less money in cash than they ever have before — meaning they are all-in on risky investments.

Confidence this solid is a warning sign of complacency. For one, gross domestic product growth is at the top end of the range that has prevailed over the past decade, so it is more likely to slip than accelerate. That should be sobering for every major power, including Trump’s America. But please, it’s not all about him. For better or worse, Mr. Trump had less impact on the global economy than most experts expected in 2017. The lesson — the inevitable rarely happens, the unexpected often does — applies as well to forecasting 2018.