B RITAIN’S POP-GUZZLERS are a surprisingly health-conscious lot. Earlier this year the country became the first where sugar-free versions of Coca-Cola outsold the real thing. Whether Britons follow similar principles when it comes to imbibing their politics will dictate the fortunes of Labour and the Conservatives.

“Diet” policies have become the norm in British politics, with both main parties promising a healthier version of the other’s platform. In last month’s budget the Conservative chancellor, Philip Hammond, announced that austerity was “coming to an end”, in what amounted to a toned-down version of Labour’s promise to open the spending taps. Meanwhile his opposite number, John McDonnell, surprised many in his party by saying that Labour would support the Tories’ plan to give a tax break to the well-off, by raising the threshold at which the higher rate of income tax is levied. The Tories have previously stolen Labour ideas such as a cap on energy prices, while Labour has adopted a law-and-order policy that would fit snugly into the Conservatives’ manifesto, with its call for more police and border guards.

Whether parties benefit from aping their opponents depends on the policy area, argues Margit Tavits of Washington University in St Louis. She examined voters’ behaviour in 23 countries over 40 years. On issues that are seen as pragmatic, such as the economy, voters are happy for a party to change its tune. But when it comes to so-called principled policies, including immigration, voters will punish a party that strays from its beliefs. This is the case even when the party is shifting towards the median voter’s views. Both Labour and the Conservatives lost ground to the UK Independence Party when they mimicked its hard line on immigration, offering stiff rhetoric but few policy changes. This pattern repeated itself across Europe.

But what works on the pages of an academic journal is messier in real life, argues Tim Bale of Queen Mary University of London. Deciding whether a policy area is pragmatic or principled is more art than science. The National Health Service, for example, bestrides both. And nabbing too many of an opponent’s policies may annoy a party’s core supporters, whatever the issues in question. Mr McDonnell faced grumbles from Labour MP s (including those usually well to the right of him) for supporting the Tories’ tax break. Some Conservative MP s complain that Theresa May spends too much time talking about what they see as Labour issues, like NHS funding, and not enough on traditionally Tory concerns, such as lower taxes.

Labour aides are confident that the Tories’ “Diet Corbynism” is no match for the real thing. After all, the government’s mooted plan to reduce tuition fees for some subjects pales in comparison to Labour’s promise to abolish them (as well as smacking of hypocrisy, since the Tories introduced the higher fees only six years ago). Conservative wonks insist that voters do not care where a policy came from. The next election will depend on whether voters find Labour’s full-fat politics more tempting than the Tories’ offering of Corbynism without calories.