THE planet has rarely been so peaceful. Even with terrible fighting in such places as Congo, Syria and Yemen, wars between and within countries are becoming less common and less deadly. But a dark menace looms. Some of the developing world’s cities threaten to be engulfed by murder.

Of the 560,000 violent deaths around the world in 2016, 68% were murders; wars caused just 18%. Murder has been falling in rich countries (though London is suffering an outbreak—see article), but it has long plagued Latin America and is starting to climb in parts of southern Africa, the Middle East and Asia. The world often goes to great lengths to stop wars. Just imagine if it put as much effort into stopping murders.

Latin America shows what is at stake. It has 8% of the world’s people but 38% of its recorded murders (see Briefing). Counting the costs of police, hospitals, victims’ lost incomes and so on, the bill for violent crime comes to 3.5% of GDP. The greater toll is human: mothers and fathers burying children, children brought up without a parent and societies deprived of tens of thousands of citizens in the prime of their lives.

There is no excuse for this suffering. Many of the emerging world’s problems are intractable. Murder is not one of them.

Dusty death

High murder rates have lots of causes: fragile government; guns and fighters left over from wars; families broken up and forced into the city by rural violence and poverty; drugs and organised crime that police cannot or will not confront; and large numbers of unemployed young men.

The mix of causes in each country is unique but in every case rapid, chaotic urbanisation makes the problem worse. Urbanisation itself is welcome, because it boosts incomes and growth. It need not lead to violence—look at India and China, which have relatively low crime rates. But it can feed a vicious cycle, as the proliferation of murder destroys trust between the police and the people they are meant to protect. Residents keep off the streets. They no longer support the authorities. Impunity grows and the level of violence climbs further.

That is what faces some of the world’s poor cities. Many already have the ingredients of a murder culture. Over the next decades these cities are set to grow rapidly. As much as 90% of urban growth will take place in the poor world. By 2030, according to HSBC, a bank, 42 of the 50 most-populous cities will be in emerging markets. Dhaka, Karachi and Lagos, each crammed with roughly 25m people, will join the ten largest.

To understand where this can lead, consider Latin America, where seven countries account for a quarter of the world’s murders. Killings often started rising in its cities because of drugs and gangs. Since Latin America urbanised a generation before other developing countries, it has had time to find out which policies help stop the killing—and which fuel it.

El Salvador, though off the main drug-trafficking corridor, has struggled to establish peace since the end of civil war in 1992. A weak state failed to cater for hundreds of thousands of new city dwellers, driven into slums by fighting in the hills. The police were unable to cope with violent new residents who arrived in the slums: street gangs like MS-13 and Barrio 18. In 2015 El Salvador became the world’s most deadly country bar Syria. The police still do not know who is killing whom or why. Ninety-five percent of murders go unsolved.

Yet the continent also has some of the biggest improvers. In many Colombian cities murder used to be the leading cause of death. The rate in Cali in 1994 was 124 per 100,000, four times worse than New York at its most lethal. The mayor was a surgeon who realised that murder was like a disease. Following an approach pioneered in New York and copied across the rich world, he set up “violence observatories” to study precisely how people, places and behaviour led to killings. They found that, even amid a raging drugs war, most murders resulted from drunken brawls. Restrictions on alcohol and guns helped cut murders by 35%. Other Colombian cities tweaked Cali’s evidence-based policing to suit their own needs—Medellín, for example, targeted drug cartels. Police and judicial reform, and aid from the United States, were crucial, too. In 2017 Colombia’s murder rate was 24 per 100,000, the lowest for 42 years.

The killing in Latin America has been shaped by local factors—political violence, crowded prisons, North America’s appetite for drugs. In Colombia an offensive against guerrillas helped cut murders. Yet the continent holds broad lessons.

Because impunity encourages murder, you might think that the secret is tough policing. The Salvadorean government sent soldiers into the streets and threw petty criminals in prison. But both the criminals and the population were brutalised and the murder rate rocketed. Or you could buy peace with truces and special deals between rival gangs. El Salvador tried that, too, but the truce fell apart and the slaughter resumed. Although murders there have fallen slightly in the past two years, killing has become a way of life.

Instead, toughness needs to be targeted. Murder is extraordinarily concentrated—80% of violent killings in Latin American cities occur on just 2% of streets. Detailed crime statistics enable the police to get to grips with the local factors behind the killing. If they know exactly how and where to apply their efforts, they can make arrests and prevent violence.

Learning from murder

The good news is that you do not have to solve all the complex social causes of murder to begin to cut the murder rate. Colombia’s innovative mayors helped create a virtuous cycle in which the police caught and punished murderers, and citizens concluded that the police were there to help them and that the streets were safer. Busier streets discouraged murder and other violent crime. Once the virtuous cycle starts to turn, the entire criminal-justice ecosystem can grow stronger.

If all countries cut murder rates to match the best in their region, 1.35m lives will be saved by 2030, says the Small Arms Survey. If they fall back to the worst, an extra 1.25m lives will be lost. Trusted police forces armed with good statistics could spare millions of lives, and an ocean of human suffering.