Conventional Wisdom May Be Contaminating Polls | FiveThirtyEight

Sunday’s French presidential election was the latest in a trend. The centrist candidate, Emmanuel Macron, won by a considerably wider margin than most observers predicted, with a 32-percentage-point landslide over Marine Le Pen, larger than the 24-point margin that the final polls showed.

But the trend isn’t that center-left globalism is making a comeback — that’s too early to say.1 Instead, it’s this: When the conventional wisdom tries to outguess the polls, it almost always guesses in the wrong direction. Many experts expected Le Pen to beat her polls. Currency markets implied that she had a much greater chance — perhaps 20 percent — than you’d reasonably infer from the polls. But it was Macron who considerably outperformed his numbers instead.

While this was somewhat amusing — the one time the experts decided to take the nationalist candidate’s chances really seriously was the time she lost by 32 points — it should actually worry you, even if you’re a “fan” of polling and data-driven election forecasting. It’s a sign that the polls may be catering to the conventional wisdom, and becoming worse as a result.

This French election was part of a pattern that I began to notice two years ago in elections in the U.S. and elsewhere in the world. Take the 2012 U.S. presidential election as an example. Most of the mainstream media concluded that the race was too close to call, despite a modest but fairly robust Electoral College lead for then-President Barack Obama. But on Election Day, it was Obama who beat his polls and not Mitt Romney.2

The 2014 U.S. midterms provided another example, only in reverse. That time, the mainstream media was full of articles suggesting that polls might be “skewed” against Democrats, perhaps because they underestimated minority turnout. Republicans beat their polls by several percentage points, however, and gained nine seats in the U.S. Senate.

The pattern also replicated itself in the three highest-profile elections around the world in the past year. Ahead of the U.K.’s vote to leave the European Union, polls showed a razor-close race, with “Remain” ahead by only a percentage point or two in a country notorious for inaccurate polling. It would have been reasonable to call the race a toss-up. And yet the London-based media was highly confident that Remain would prevail and bookmakers assigned Remain about a 90 percent chance on the day of the vote. It was “Leave” that won instead, of course, by 4 percentage points.


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