In the past, the traffickers perhaps did more to help the residents and less to harm them. In Nova Hollanda, one of 17 favelas that make up the Complexo da Maré (combined population: 135,000), Ana Caroline, 24, tells me: "It used to be that the bandidos would hide their guns if an old lady walked past, or they wouldn't smoke weed in front of children. But now they don't care." Natalia, 16, confirms: "The new guys are trying to be cool in bad ways." Squatter settlements are monuments to, among other things, the instinctive ability of a group of human beings under a common hardship to adapt, survive and progress as one. Rocinha has received more state help than most favelas because of its sheer size and its prize location in Rio's bourgeois South Zone, but it would have become a fashionable neighbourhood under its own steam by now if it were not for the bandidos. Even in the most neglected favelas in the suburban flatlands, where residents are additionally crippled by the massive distances they have to travel to work, things improve year by year. What starts as cardboard gradually turns to wood, and eventually becomes brick. But the tendency of the criminal element is to move, over generations, in the other direction, towards what William Langeweische, writing in Vanity Fair in 2007 about the "First Command of the Capital" (Sao Paulo's conspiratorial prison mafia, in fact more powerful and terrifying than any of Rio's factions), dubbed the "feral zone" — a world of increasing violence and increasingly arbitrary demonstrations of its power.

Fernando (not his real name), 20, from Rocinha, explained to me the rules by which residents of the favelas live. You aren't allowed to rob, kill or threaten another resident. Murder is almost always a capital offence, but breaking the other rules might result first in a warning (optionally reinforced by light torture), and then death at the second offence (optionally reinforced by heavy torture). Punishments include being beaten with spiked bats, or being thrown into, and forced to drink from, the rudimentary sewage channels that run through many favelas. The residents risk this kind of treatment when they enter a favela other than the one in which they live. You are not allowed to rape, in the sense of sexually assault, another resident.

We're standing on a Rocinha street with Fernando's mother and three-year-old niece, the beautiful Maria Clara, who is perched on a windowsill, legs daintily crossed, eating an ice lolly. "Are you wearing a bikini under your dress?" she asks my friend, who confirms that she is. "Why? I never wear a bikini unless I'm going to the beach. I have a swimming pool too, but it's just for me." Across the street a skinny mulatto with bleached hair is cooling his heavily tattooed and heavily scarred torso under a hosepipe. Fernando and his mother warn us that he is an estrupador, "a guy who takes a woman by force". People complained to the traffickers, and he got kicked out of the favela a couple of months ago, but now he has reappeared. Later, Fernando tells me that he was killed that very day: "It was a miracle they even let him go in the first place, but they didn't give him a second chance. Near the top of the favela, there's a place where they bury or burn the bodies."

Statutory rape, on the other hand, is not just allowed but actively encouraged by the bandidos — sleeping with very young girls is central to their culture. One of Fernandos's neighbours is a pregnant 11-year-old. When her father went to complain to the dad-to-be, a 17-year-old bandido, he responded simply: "What do you want from me? It's not my fault she opened her legs for me." Fernando says the parents are partly to blame. Often barely out of childhood themselves, "they encourage their daughters to wear sexy clothes". Rio is a sexy city, and the favelas are the sharp end of it. And Ana Caroline and Natalia, the two girls I interviewed in Nova Hollanda, are untypical: often the young girls are the bandidos' biggest fans. It's as Patricia Melo describes it in Inferno, her brilliantly funny, visceral novel about the favelas: "At 13 or 14, the girls were already showing their asses and looking for trouble...And they were only interested in armed men. The bigger the weapon, the greater the guy's chances."

Every one of Fernando's five sisters has been "a big, big, disappointment" to him. One of them, 17, complained to their mother that the bandido who made her pregnant had absconded. Dona Maria remarked in an offhand way that he was probably getting drunk somewhere. When the bandido got wind of this he came into Dona Maria's house, drunk, with a pistol, and beat her — for "putting it about" that he was a drunk. (Fernando went to the traffickers for another service they provide, the settling of disputes and the sanctioning of fair fistfights. They said, "You're right, beat him up." So Fernando did.) Another sister had a non-criminal husband who they were all very fond of, but she cheated on him with a bandido. Maria Clara's mother now lives in the Cidade de Deus (City of God) favela. Her father is a well-known killer from Rocinha. He lives somewhere else now, but can be called upon by the Amigos dos Amigos if his services are needed. Whether Maria Clara can ever call on him is much less certain. Once, when Dona Maria was doing laundry, a hand grenade fell from his dirty clothes.