Which Western European country has sent the most fighters to Syria and Iraq?

In a recent article entitled 'Denmark's "failed" multiculturalism', The Economist writes:

The generosity of the country’s social-security net has made people quicker to accuse immigrants of exploiting it. Meanwhile, a highly cohesive and demanding civic culture, which was the basis of Denmark’s liberal social-welfare policies, has alienated anyone who finds it hard to conform. This has made the country fertile ground for Islamist recruitment. Over 100 Danish jihadists have gone to Syria and Iraq, one of the highest rates per person in Europe.

Indeed, the International Centre for the Study of Radicalisation and Political Violence estimates that 100-150 fighters have gone to Syria and Iraq from Denmark, corresponding to the second highest per capita rate in Western Europe after Belgium. Yet these per capita rates do not take into account the size of the Muslim population in each country, which is arguably a more appropriate denominator. Using population data for 2013 from the World Bank, and the fraction of the population estimated to be Muslim from Pew Research, I calculated the number of fighters per Muslim for each of the Western European countries in the ICSR dataset. The first chart uses the low estimates for the number of fighters, and the second chart uses the high estimates.

In both cases, Finland has the highest rate of fighters per Muslim, whereas Denmark has only the fourth highest. Thus, when the Muslim population is taken as the denominator rather than the total population, Denmark is seen to have sent slightly fewer fighters to Syria and Iraq relative to some other Western European countries, notably Finland and Ireland. And this is because Denmark has a larger Muslim population (4.1%) than both Finland (0.8%) and Ireland (0.9%).

Several caveats are of course in order. First, the Pew figures on fraction estimated to be Muslim, which are form 2010, may be out of date. Second, an even more appropriate denominator might be the Muslim population aged 15-30, or better yet the male Muslim population aged 15-30. Third, although it is presumed that the majority of those who have gone to Iraq and Syria are Muslims, a small number are evidently not. Fourth, differences in the number of fighters across countries reflect factors other than how well integrated the Muslim population is; for example, how stringent the security services have been in interdicting those trying to leave.