FEW South Sudanese see a link between their country’s horrific civil war and polygamy. Instead they blame greedy politicians or the tribe next door. Fair enough: corruption, weak institutions and tribalism all make violence more likely. But marital customs matter, too. Wherever polygamy is widely practised (in South Sudan, perhaps 40% of marriages involve multiple wives) turmoil tends to follow. The 20 most fragile states in the world are all somewhat or very polygamous. Polygamous nations are more likely to invade their neighbours. The polygamous regions of Haiti and Indonesia are the most turbulent. One London School of Economics study found a strong link between plural marriage and civil war. How come?

Polygamy nearly always means rich men taking multiple wives. And if the top 10% of men marry four women each, then the bottom 30% cannot marry at all. This often leaves them not only sexually frustrated but also socially marginalised. In many traditional societies, a man is not considered an adult until he has found a wife and sired children. To get a wife, he must typically pay a “brideprice” to her father. When polygamy creates a shortage of brides, it massively inflates this brideprice. In South Sudan, it can be anything from 30 to 300 cattle, far more wealth than an ill-educated young man can plausibly accumulate by legal means.

In desperation, many single men resort to extreme measures to secure a mate. In South Sudan, they pick up guns and steal cattle from the tribe next door. Many people are killed in such raids; many bloody feuds spring from them. Young bachelors who cannot afford to marry also make easy recruits for rebel armies. If they fight, they can loot, and with loot, they can wed. In a paper published last year, Valerie Hudson of Texas A&M University and Hilary Matfess of Yale found that a high brideprice is a “critical” factor “predisposing young men to become involved in organised group violence for political purposes”. Jihadist groups exploit this, too. One member of Pakistan’s Lashkar-e-Taiba, which carried out the attack on Mumbai in 2008 that killed 166 people, said he joined the organisation because it promised to pay for his siblings to get married. During its heyday the so-called Islamic State offered foreign recruits honeymoons in Raqqa, its former capital. In northern Nigeria, where polygamy is rife, Boko Haram still arranges cheap marriages for its recruits.

Globally, polygamy is in retreat, but in some pockets support for it is rising. After America’s Supreme Court legalised same-sex marriage in 2015, some people argued that plural unions should be next. According to Gallup, a pollster, the proportion of Americans who consider polygamy to be morally acceptable rose from 5% in 2006 to 17% last year, among the most dramatic jumps in the subjects it tracks. Campaigners in Kyrgyzstan, Turkmenistan and other central Asian states are seeking to re-establish men’s right to take multiple wives. In Kazakhstan, a bill failed in 2008 after a female MP included an amendment stipulating that polyandry (women taking multiple husbands) also be allowed. Advocates claim that polygamy promotes social harmony by giving lusty husbands a legitimate alternative to infidelity. The mayhem in places like South Sudan, Afghanistan and northern Nigeria suggests otherwise.