Those numbers are a recipe for striking imbalances within Sweden. Consider that more than half of these unaccompanied minors entering Sweden are 16 or 17 years old, or at least claim to be. (There are no medical checks of age for Swedish asylum-seekers, and applicants who say they’re under 18 receive special consideration in the asylum process.) In this age group more than three-quarters are unaccompanied, meaning they are overwhelmingly male. According to calculations based on the Swedish government’s figures, a total of 18,615 males aged 16 and 17 entered Sweden over the course of the past year, compared with 2,555 females of the same age. Sure enough, when those figures are added to the existing counts of 16- and 17-year-old boys and girls in Sweden—103,299 and 96,524, respectively, according to the U.S. Census Bureau’s International Database—you end up with a total of 121,914 males in Sweden aged 16 or 17 and 99,079 females of the same age. The resulting ratio is astonishing: These calculations suggest that as of the end of 2015, there were 123 16- and 17-year-old boys in Sweden for every 100 girls of that age…

Fear of terrorism could well be part of Canada’s calculus, especially in the wake of attacks perpetrated by migrants in Europe and the United States; in the overwhelming majority of cases, terror attacks are carried out by unattached young adult men. Most of these men are unmarried, and virtually none have children. Indeed, the Islamic State reportedly discourages its male fighters from having children so that they are more willing to engage in suicide attacks, and widows of suicide bombers are quickly forced to remarry, while remaining on birth control.

But fear of terrorism might not be the only reason to be leery of highly abnormal sex ratios among the young adult population. As my co-author Andrea Den Boer and I argued in our book, societies with extremely skewed sex ratios are more unstable even without jihadi ideologues in their midst. Numerous empirical studies have shown that sex ratios correlate significantly with violence and property crime—the higher the sex ratio, the worse the crime rate. Our research also found a link between sex ratios and the emergence of both violent criminal gangs and anti-government movements. It makes sense: When young adult males fail to make the transition to starting a household—particularly those young males who are already at risk for sociopathic behavior due to marginalization, a common concern among immigrants—their grievances are aggravated.