Alongside the result itself and controversies surrounding the campaign, one of the biggest stories of the London Mayoral election was its remarkable turnout.

Despite predictions of low turnout in polls, 46 per cent of the electorate cast a vote in 2016, up from 38 per cent in 2012. The 2016 turnout is the highest recorded since London was granted the right to elect a mayor in 2000, slightly over the 45 per cent mark when Boris Johnson first ran in 2008.

Commentators have put forward a range of explanations for this unexpected surge in democratic activity: the appeal of a guaranteed new mayor, an emotional reaction to the main candidates’ campaigns, or as a sign that the mayoral institution is growing in significance.

Others, however, have suggested that the increase in turnout reflected electoral roll shrinkage, as a result of the new voter registration system introduced in 2014. Voters now need to register individually, rather than being registered by the household. Young people, as renters, were predicted to lose out in the transition.

Was the increase in turnout a mark of renewed civic enthusiasm? Was it the effect of London’s rapid population change? Or was it the direct consequence of having fewer voters on the register?

The truth is: it was a mixture of the three.

Population change

The background to the election, like so many things in London, is the city’s growth. London’s population is growing, and changing as it grows, which could explain why Londoners are more likely to cast a vote.

The capital is gradually becoming home to a greater proportion of higher-skilled people, who are more likely to vote. London has also gained 350,000 people in voting age between 2012 and 2016. (We use adult population as a proxy for the number of people who have the right to vote in the London.)

Population increased most rapidly in the East End – where it grew by a striking 10 per cent in just four years – but more slowly in west London boroughs. (Figures are for the mayoral election and boroughs are grouped together by London Assembly constituency.)

Electoral register

The adult population has grown by 350,000 – yet the electorate has shrunk. Indeed, the electoral register in May 2016 had 60,000 fewer people than in 2012. The registration rate dropped from 90 to 84 per cent of the adult population.

This drop in registration may be attributed to the new registration system. This requires individuals to take action. With fewer people registered by default, you would expect the smaller electorate to be more likely to turn out on polling day. 

The drop in the electorate was most dramatic in the West End. This area has seen most growth in international buyers of prime London real estate, who may not be able to vote or may not even live in the capital. The Guardian recently reported that 184 of the 214 apartments in St George’s Wharf in Vauxhall had no person registered to vote.

*May 2012 data for Croydon and Sutton and Bexley was wrong – for these two constituencies December 2012 data was used instead.

Votes cast

But turnout also increased because more people voted in 2016 than in 2012, or even in 2008, despite having fewer voters on the electoral roll. The number of people who turned up to their polling station went down from 2.4m in 2008 to 2.2m in 2012, then up again to 2.6m in 2016.

As the table below shows, the number of people who voted increased most in East London, which has gained a lot of residents. There were also large increases in the two frontrunners’ constituencies, South West London and Merton & Wandsworth; and in the boroughs neighbouring Heathrow Airport, where runway expansion may have galvanised residents.

Here’s those figures again, this time as a share of the total adult population.

This 18 per cent increase in the number of people who voted means that turnout of the voting age population – Including those Londoners who did not register – grew from 35 per cent to near 40 per cent, a similar level to 2008.

This is mixed news for the advocates of elected Mayors. Turnout of the voting age population did come up in the 2016 London election – but just to bounce back to its 2008 level. More people voted than in previous elections, but the growth in the electorate could have been even higher if registrations had kept up with population growth.

With the EU referendum round the corner, London’s mayoral elections have perhaps helped make sure that the capital will turn out to vote. But a growing population with a shrinking electoral roll is not a healthy sign.

Nicolas Bosetti is a research intern at the Centre for London. He tweets as @nicolasbosetti.

Want more of this stuff? Follow CityMetric on Twitter or Facebook.