There were so many of them: weary young men, some barely teenagers, trekking across Europe to reach the promised German land. I remember wondering a few months back, during the peak of the Syrian migration wave, why the women had been left behind.

There were reasons men sought refuge first: the arduous journeys, the pressing need for work before applying for family reunification and, above all, escape from recruitment by the army or militias. It is not unusual in war for parents to send their boys away.

Yet the impact of this gender imbalance was a largely overlooked aspect of the migration crisis. In Angela Merkel’s remarkable drive to show compassion for a people the world had tried so hard to ignore, some risks were understated. It seemed insensitive and politically disadvantageous, in the face of opposition to the migration surge, for supporters of the German chancellor’s humanitarian policy to dwell on the consequences.

After the ghastly New Year’s Eve in Cologne, however, questions that should have been raised and vigorously debated in public are finally being voiced. The details of that night are murky but we know enough — and the scale of the attacks is shocking. The number of complaints filed by women has been gathering pace, 40 per cent of them related to sex attacks.

How many asylum seekers, whether from Syria, Afghanistan or Iraq, were involved has yet to be established. But they may have been recruited by north African gangs responsible for many robberies in the city in recent years.

Scholars who have studied mass migration of young men and discovered a correlation with a rise of crime and attacks on women are unsurprised by this turn of events.

Much has been made of the demeaning attitude towards women that some of Europe’s newest Muslim migrants may have grown up with. But Valerie Hudson, a professor of political science at Texas A&M University who has researched migrant issues in Asia, says the sex ratio is far more important than different interpretations of female modesty.

“The literature I’ve contributed to shows a pattern: the higher the sex ratio, the higher the crime rate and crimes against women,” she tells me. “When you get a surplus of young men in a society — and they are marginalised, disadvantaged, and they live together and socialise together — you have the beginnings of collective activity in which they take what society has denied them. And they are, collectively, willing to take risks.”

Such fears about sex ratios in cases of mass migration extends beyond the immediate consequences. Many young male migrants are minors, which facilitates their asylum applications but could have a longer lasting impact on the sex ratio in the host country.

Andrea Den Boer, an expert on gender imbalance in Asian internal migration from the UK’s University of Kent, says there are already measures preventing asylum seekers who arrive as unaccompanied minors from bringing their families over at a later stage — which in turn further inflates the proportion of young men in the population. Some work has been done on the potential long-term impact in Sweden but there are no official statistics on the ages of the refugees to Germany. Den Boer’s rough estimate is that 72 per cent of refugees last year were male, but no one knows how many were young adults. “Nobody has thought about the sex-ratio implications,” she says.

The one country that has taken gender into account is Canada, where the government said last year that it would take only Syrian women, children and families. The policy was probably prompted by concerns over terrorism — and it drew its share of critics who warned that young men faced the greatest risk in Syria.

There are no easy answers to the mass migration dilemma, particularly where horrific crimes are being perpetrated on an innocent population as in Syria. But taking into account the long-term implications for the host society should be an integral part of any policy. As Prof Hudson says: “A normal sex ratio is a public good.”

— Financial Times