The recent surge of migration into Europe has been unprecedented in scope, with an estimated 1 million migrants from the Middle East and North Africa this past year alone, making for a massive humanitarian crisis, as well as a political and moral dilemma for European governments. But one crucial dimension of this crisis has gone little-noticed: sex or, more technically, sex ratios.

According to official counts, a disproportionate number of these migrants are young, unmarried, unaccompanied males. In fact, the sex ratios among migrants are so one-sided — we’re talking worse than those in China, in some cases — that they could radically change the gender balance in European countries in certain age cohorts.

As many governments, including in the United States, debate how many migrants to accept onto their shores, they would be wise to take gender balance into consideration. That might sound sexist on the surface, but years of research has shown that male-dominated societies are less stable, because they are more susceptible to higher levels of violence, insurgence and mistreatment of women. In Germany, scores of women recently reported being attacked on New Year’s Eve by men whom the authorities describe as of “North African or Arabic” descent. While it is not yet known whether the alleged perpetrators were migrants, the attacks may finally be alerting policymakers to the risks of a male-dominated migration wave. Why would European societies, many of which rank highest on global measures of gender equality and stability and peace, jeopardize those hard-won and enviable rankings?

It makes good sense that so many young men are leaving countries like Afghanistan, Iraq and Syria: Their demographic is often at greater risk of being coerced into joining fighting groups, or being killed rather than captured by such groups. But the result is that 66.26 percent of adult migrants registered through Italy and Greece over the past year were male, according to the International Organization of Migration.

That imbalance might not sound radical, but it is, especially when you look more closely at who those males are. It’s true that many male migrants hope that, if granted asylum, they will be joined in Europe by their wives and children, who would help balance out national sex ratios. But importantly, more than 20 percent of migrants are minors below the age of 18, and the IOM estimates that more than half of those minors traveling to Europe are traveling as unaccompanied minors — 90 percent of whom are males. This heavily male subset is all but guaranteed asylum because of their status as unaccompanied minors, but they get no special dispensation to bring spouses, especially since the European Court of Human Rights recently ruled that European Union countries are not required to recognize the legality of child marriages among migrants.

To see how these overall figures affect specific countries — and why there is reason for concern — consider the case of Sweden, which has been especially transparent about its migration statistics and whose ratios mirror the broader trend in Europe in many respects.

According to Swedish government statistics, as of the end of November, 71 percent of all applicants for asylum to Sweden in 2015 were male. More than 21 percent of all migrants to Sweden were classified as unaccompanied minors, representing more than half of all minor migrants to the country. For accompanied minors, the sex ratio was about 1.16 boys for every one girl. But for unaccompanied minors, the ratio was 11.3 boys for every one girl. In other words, the Swedish case confirms IOM’s statistic that more than 90 percent of unaccompanied minors are male. Indeed, on average, approximately 90 unaccompanied boys entered Sweden every single day in 2015, compared with eight unaccompanied girls.

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.

If that trend continues into 2016 or even beyond, each successive late adolescent cohort of 16- and 17-year-olds will be similarly abnormal, and over time the abnormality will become an established fact of the broader young adult population in Sweden. (Hans Rosling, the Swedish data visualizer who created the GapMinder Foundation, has similar estimates regarding the alteration of Swedish sex ratios.) In China, long the most gender-imbalanced country in the world, the male-to-female ratio of approximately 117 boys for every 100 girls in this age group now comes up short of Sweden’s gender gap. China’s sex ratios are still more abnormal across other age groups; the imbalances there extend all the way down to birth sex ratios due to the country’s severe birth restrictions, while Sweden’s abnormalities do not. But young adult sex ratios are arguably the most crucial of all for social stability.

Canada is the one country so far that seems to think this is cause for concern. Faced with similarly skewed sex ratios among asylum-seekers, the new liberal administration of Justin Trudeau announced in late November that, starting in 2016, it would accept only women, accompanied children and families from Syria. Specifically excluded would be unaccompanied minor males and single adult males (unless they are members of the LGBTQ community); those excluded will primarily be older teen and young adult men.

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.

There are also clearly negative effects for women in male-dominated populations. Crimes such as rape and sexual harassment become more common in highly masculinized societies, and women’s ability to move about freely and without fear within society is curtailed. In addition, demand for prostitution soars; that would create a deeply ironic outcome for Sweden, which invented the path-breaking Swedish abolitionist approach to prostitution.

Europe is famously progressive on women’s rights, and some European governments have even created voluntary classes for migrants to understand how the treatment of women may be profoundly different in their new homes. But even with such efforts there is the potential for real regress when the young adult sex ratio is so high. And what is often invisible in the debates over migration is that the women left behind by this largely male exodus are usually left in dire situations: In displaced persons camps in Syria or refugee camps in Turkey, Jordan and surrounding countries, female-headed households live in fear and penury, prey to exploitation and abuse. Sweden’s foreign minister, Margot Wallström rightly emphasizes her country’s “feminist foreign policy”—but can Sweden really consider its migration policy to be feminist?

While the humanitarian needs of the refugees streaming into Europe must be foremost in our minds at this time, policymakers in Sweden and other countries should also think of the long-term consequences of an unprecedented alteration in the young adult sex ratios of their societies. The Canadian approach should be carefully studied, and perhaps adapted by other countries. After all, if the sex ratios of the migrants’ countries of origins are balanced, is it not odd to accept predominantly male migrants for asylum?

As anthropologist Barbara Miller has persuasively argued, a normal sex ratio is a “public good” and therefore deserves state protection. For Sweden—or any other European country—to wind up with the worst young adult sex ratios in the world would be a tragedy for European men and women alike.

Valerie Hudson is professor at the Bush School of Government and Public Service at Texas A&M University and co-author, most recently, of The Hillary Doctrine: Sex and American Foreign Policy.