W ITH AN ELECTION to win in 2020 Sadiq Khan, London’s mayor, is sniffing around for popular policies. He has chosen to woo renters. On January 23rd he announced that he would develop a “blueprint for stabilising or controlling private rents in the capital”. One in four Londoners rents privately. On average they send over 40% of their monthly pre-tax income the landlord’s way, a far higher share than in the rest of the country. Mr Khan’s policy is likely to prove popular: more than two-thirds of Londoners are in favour of rent controls. But would it be effective?

The mayor is riding the crest of a rent-control wave. Four years ago some German cities introduced controls in areas where rents were deemed too high. In 2017 Scotland gave local councils the power to limit rent increases on certain private tenancies. In November a plan to control rents in California was put to voters in a referendum (it failed). Proposals for rent control appeared in the most recent manifesto of the Labour Party, to which Mr Khan belongs.

The mayor does not have the power to control rents in London, so changes to legislation would be required. Unfortunately for him, the ruling Conservative Party is not keen on rent controls. And understandably so. The age of rent control in Britain, which lasted from 1915 to 1989, was not a glorious one. Some properties had their rents fixed in cash terms from 1939 to 1957, resulting in a real-terms fall of 60%. As landlords’ returns dwindled they skimped on repairs and upgrades. Many pulled out of the market. The decision in the late 1980s to liberalise rents breathed life back into the market. In the past 30 years investment in dwellings has risen smartly.

A case can be made that putting limits on rent rises in the capital now would not cause as much damage as it did in the past. With Brexit uncertainty mounting, London’s private rents have been flat in nominal terms for a year (and have fallen in real terms). For as long as that trend continues any cap would not be tested.

The details of Mr Khan’s policy are not yet clear, but he is unlikely to propose the bluntest sort of rent control, in which clipboard-wielding officials march from property to property and tell landlords how much they are allowed to charge. Labour’s nationwide plans offer some clues as to what the mayor might propose to do in London. The party appears to favour allowing landlords to charge whatever they like when a tenancy is newly listed. But landlords would then face tough rules as to how much they could raise rents once a tenancy was under way.

That approach might seem more defensible. A landlord has a degree of market power over a sitting tenant because it is a big hassle for a tenant to move out (it is also a bother for a landlord to lose a tenant, but usually less so). Stopping unscrupulous landlords from levying above-market rent rises on vulnerable tenants is appealing. Yet the problem does not seem widespread. Evidence is sparse but data from 2015 suggest that half of renewing tenants were given the same rent on their new contract.

Were the cap more aggressive, the impact on the market would be larger. Labour envisages increases in line with inflation. Before the Brexit-related slowdown, rents in London rose only slightly faster than prices. But were a wedge to open between market rents and what was permitted, more incumbent tenants would stay put. Landlords, meanwhile, would want them to leave, so might provide a worse service. When possible they might sell up. A recent study of limits on rent increases and evictions in San Francisco found that the policies decreased the supply of rental housing, causing a 5% city-wide rent increase.

There are other ways to help renters. Many have seen their housing costs jump as a result of recent changes to welfare—most notably, a cash-terms freeze on housing benefit that has been in place since 2016. Building more houses would help, too. In 2016-17 Mr Khan exceeded his target for overall housing completions, yet fell well short of the one for “affordable” (ie, state-subsidised) dwellings. Pledges to control rent get the headlines, but the unsexy policies matter a lot more.