As the territory controlled by ISIL has shrunk, former fighters have begun facing up to the end of their so-called caliphate. Some have been captured and are held in jails in Syria, Iraq and Turkey. Others have fled, returning to their home countries, going to ground in Syria or elsewhere or migrating to other conflicts.
Their home countries are also facing a dilemma about what to do with their citizens if and when they return. An estimated 40,000 foreigners from 110 countries have gone to Iraq and Syria to fight with ISIL – yet there is no clear consensus on what to do with those who have been captured or have returned. In some cases, there will be evidence for prosecution. But given the nature of the warzone, in most cases there will not be.
Most governments would prefer to avoid the problem entirely, by seeing their citizens killed on the battlefield or removing their citizenship. Britain, Australia and France have all taken those steps. European countries can't agree what to do but the US defence secretary Jim Mattis has said: "Countries of origin keep responsibility for them".
Returning foreign fighters are one of the most pressing political problems; countries from Europe to Russia to the Middle East to southeast Asia have sought to use military, security and political levers to solve it. But instead of looking exclusively at these options, it is worth exploring how innovative economic levers could be used to make the problem more manageable. Surprisingly, the dismal maths could actually help stem the flow of foreign fighters.
The essence of the problem is that those returning to their home countries fall somewhere on a spectrum of radicalism but the authorities have no way of knowing where. There will be, among the thousands, those who are committed to the ISIL ideology and who are seeking to radicalise others or carry out violent attacks; there will be those who might have believed in the ideology but have become exhausted and want to return to a different life; and there will be those who are genuinely penitent or disillusioned or who, in the end, took little part in the atrocities of ISIL.
But in the absence of hard evidence it is impossible to know. Fighters have every incentive to pretend they were low-level functionaries, cooks or administrators. Surely no-one would admit to being a senior ISIL fighter on their own?
One way to sort through the returnees is to use signalling, a economic technique whereby customers give information about themselves – or send “signals” – to companies, thereby allowing the companies to decide how much to charge each customer.
A simple example is the packaged, already chopped fruits supermarkets sell, at a higher price than the identical but unpackaged and uncut whole fruit beside it. Customers who buy the pre-cut fruit are sending a signal that they value convenience and are willing to be charged more.
Countries can apply some of this theory to foreign fighters to discover which are committed radicals and which merely fellow travellers.
For example, a government could mandate that anyone who admitted or was proven to be in Syria or Iraq with ISIL would face a mandatory three-year jail sentence in their home country and be made to attend deradicalisation classes. In return, if there were no evidence against them, they would be released at the end of their sentence.
That would immediately sort some of the fighters. Those who were genuinely penitent would see three years in jail (or however long the sentence is) as a small price to pay for restarting their lives and returning to their families. Others who took the offer reluctantly could still have their minds changed by the deradicalisation process.
It would only be those who remained committed to the ideology and were seeking to do harm in their home countries who would feel that three years in jail was too long, not give themselves up and instead seek to migrate elsewhere. This technique also has the advantage that anyone who was found to be sneaking into the country without taking the offer could be assumed to be a more committed fighter – otherwise why not take the deal?
But wouldn't high-level fighters who were penitent but guilty of serious crimes accept this offer, thereby allowing them to evade justice? They might be willing to spend three years in prison but would not be willing to admit attacks or give information, thereby depriving the intelligence services of what they know.
Again, economics and specifically game theory can help. A government could offer amnesty against future prosecution for crimes committed in Syria in return for confession and information today. The “price” would be higher – perhaps five or six years in jail.
Fighters would then have to make a decision. If they opted for the lower “offer” of three years in jail, they would run the risk that at some point in the future more evidence emerges – from the vast quantities of video on social media, a recovered hard drive or from testimony from someone else – and they would then be prosecuted for those crimes. Or they could take the higher “offer” of six years in jail and tell investigators what they know in return for amnesty.
No-one who did not commit serious crimes would take the higher offer and make up fake information; they would be spending a further three years in jail for no benefit. But plenty of serious ISIL criminals would take up the higher offer, either because they fear evidence would emerge, or simply because they fear that some other fighter would offer testimony against them in a bid for their own freedom.
Taken together, these techniques would allow the fighters to sort themselves into categories of guilt and give governments some badly needed information. It is not a perfect system – as Mr Mattis said last week, there isn't one way forward for foreign fighters – but smart use of economics can help chip away at the problem, in tandem with political and military methods. The dry study of numbers can help fix one of the hardest problems of a bloody war.
