On Dec. 12, Britain’s Labour Party suffered a heavy general election defeat. Its final tally — 202 out of 650 parliamentary seats — was its lowest since 1935. Observers have offered several explanations for the ruinous result.

One obvious suspect is Brexit, the withdrawal of Britain from the European Union. Labour’s initial attempts to find a compromise position left many voters unclear about where the party stands. The party supported holding a second referendum, a position that infuriated many of the party’s working-class base who’d voted Leave in 2016.

But Brexit isn’t the only reason. To be sure, Labour’s vote share fell disproportionately in areas of the country that supported leaving the European Union. But its support fell virtually everywhere. It wasn’t only Leave voters who deserted Labour. Many voters, whatever their stance on Brexit, rejected Labour as a credible option in this election.

Some observers link this lack of credibility to Labour’s ideological extremism under leader Jeremy Corbyn. They argue that the party’s left-wing manifesto undermined its credentials as a party capable of governing.

Our research supports that argument, showing that more moderate parties are seen as more competent — even by voters who are not especially moderate themselves.

Here’s how we did our research

Does ideological moderation deliver electoral success? Many observers of British politics assume that it’s so, treating it as a truism. The usual explanation is that moderation takes a party closer to the position of the average voter.

In a recent article in the American Journal of Political Science, we suggest another explanation. We investigated whether voters suspect that more moderate parties will be more competent in office, perceiving such parties as more realistic about what can be achieved, readier to compromise, and offering less simplistic solutions for complex political problems.

To find out whether ideological moderation does indeed convey competence, we used a technique known as conjoint experiments. These experiments were included in an online survey of a representative sample of 2,000 British adults, conducted by the fieldwork agency Deltapoll. This image from our experiments shows how it works.

Survey respondents were shown pairs of party profiles that varied randomly on eight characteristics. One of these was ideological position, which had seven categories ranging from “very left wing” to “very right wing.” Participants were then asked to choose which of the two parties they thought would be generally the most competent.

Of the eight features in these party profiles, ideological moderation was the second-most important factor in competence judgments — only unity/division proved to have more influence. Other things remaining the same, parties described as “fairly” and “very” left- or right-wing were significantly less likely — by 9 and 15 percentage points respectively — to be chosen as the most competent than parties described as in the “center.”

This pattern was basically symmetrical, with parties on left and right punished equally for drifting too far from the center ground. But parties described as “center left” or “center right” did not suffer a competence penalty. Voters clearly on the left or right tended to prefer a party on their side — but to regard a moderate party on their side as the most competent.

In this year’s election, Labour’s extremist image was a problem

This mattered in 2019 because Labour was widely seen as a very left-wing party. Party leader Jeremy Corbyn had long belonged to the party’s radical left. On his surprise election in 2015, the party platform began an unmistakable move in that ideological direction.

The electorate noticed. The British Election Study Internet Panel survey asks where respondents would place Labour on a 0-10 left-right scale; Labour’s mean position has been drifting leftward since Corbyn’s election. By the most recent published wave, in summer 2019, fully 30 percent of respondents positioned the party at the leftmost point. Over half placed it at 0, 1 or 2 — squarely in the “very” or “fairly” left-wing range that damages perceptions of competence.

Not everyone tracks parties’ movements along an abstract ideological spectrum, of course; those who do tend to be a more attentive minority. In our experiments, however, moderation paid off even among voters admitting that they had limited understanding of left and right — probably because they do understand and respond to straightforward terms like “extreme” and “moderate.” In a poll three weeks before the election, 50 percent of voters described Labour as “extreme.” The corresponding figure for the Conservatives was 33 percent.

That same 17-point gap — 46 percent (Conservative) to 29 percent (Labour) — showed up in the proportion considering the two parties “fit to govern.” Our research suggests that that is hardly a coincidence. According to British polling expert John Curtice, this was key to Labour’s defeat: “People did not think it was capable of delivering and governing effectively.”

While many of the party’s individual policies were popular as well as radical, Labour included so many of them in the same manifesto that the overall program looked undeliverable. This echoes our additional finding that parties described as less ideologically moderate were seen by respondents as less “realistic about what can and cannot be achieved.” Radical change is difficult and risky, while moderate parties can keep things ticking over.

Of course, whether a party is moderate or extreme is subjective. The parochial nature of British political discourse means that policies mainstream elsewhere in Europe are often dismissed as radical in the United Kingdom. Politicians and the media, rather than voters, are often the ones who decide what counts as realistic and reasonable. It’s certainly possible that the media constructed Labour’s reputation as an extreme or radical party, as at least one Labour MP charged.

Yet regardless of where this narrative came from, the British public came to believe it. That dented the party’s competence ratings, contributing to Labour’s devastating defeat.

Rob Johns (@robjohns75) is professor in politics at the University of Essex.

Ann-Kristin Kölln (@AnnKristinKolln) is an associate professor of political science at Aarhus University.