I N RECENT YEARS the Mexico City marathon has caused crowding on the city metro. That is not just because the city shuts down numerous roads above ground for the 42-kilometre race. It is also because cheating marathoners have been known to hop on for a quick detour to the finish line. Last year 5,000 of the 28,000 runners who finished were disqualified. Hundreds more were kicked out mid-race. No other race admits to stripping so many competitors of their places. Ahead of this year’s event, on August 25th, organisers are hoping for scurrying without skulduggery.

Most of the corredores de chocolates (Mexican slang for fake runners) are easy to spot. Each runner carries a chip across electronic checkpoints placed along the course. Those who skip to the end are doomed to disqualification—but only days later, well after they receive their medal and the crowd’s adulation. Over the past six years marathon medals have each been emblazoned with a letter. Collectively, they spell out “Mexico”. That has motivated some people to cheat and complete the set, says Javier Carvallo, the Mexico City marathon’s chief. This year a new series of six medals, which together will make up a map of the city, begins.

Other cheaters give their bib to a speedier “bib mule” before the race, hoping to attain a time in their name that qualifies for the prestigious Boston marathon. In 2017 organisers disqualified a man for wearing a bib registered to a woman called María. Others still enlist multiple runners for the race who treat the bib like a relay baton. Organisers track them down by reviewing interval times to see if a runner’s speed is unrealistically quick or varies suspiciously.

Many Mexicans think that paying the 650 peso ($33) entrance fee gives them a right to run the race however they like, says Mr Carvallo. In 2007 Roberto Madrazo, a former presidential candidate, was disqualified from the Berlin marathon after cutting a third of the course. Mr Madrazo insisted he had never intended to run the whole race. Social media can warp behaviour. Those who broadcast their preparation for the race grow desperate to post a triumphant selfie after it. But boasts on Facebook and Instagram are risky, Mr Carvallo says. Clutching a medal without a bead of sweat can lead to disqualification. Serious runners and jealous friends enthusiastically dob in suspects to organisers.

All this tomfoolery dents the reputation of the marathon, which is among the fastest-growing in the world. It also affects the contest. Die-hard runners expect clear streets only to find them full of plodding course-cutters, says Derek Murphy of Marathon Investigation, a blog that dashes after the bad sports of marathons around the world. This year, an ad campaign is promoting honesty. For the first time, cheaters will be banned for life. That might be enough to keep los chocolates at bay.■