As millions of asylum seekers flee the deepening crisis in the Middle East, an American researcher has urged Europe to preserve natural gender balances when considering which refugees to accept.

Key points: 58 per cent of Europe's arrivals in 2015 were adult men, UNHCR data says

58 per cent of Europe's arrivals in 2015 were adult men, UNHCR data says Sweden has 123 boys for every 100 girls, more significant difference than China under one-child policy

Sweden has 123 boys for every 100 girls, more significant difference than China under one-child policy Research shows where men outnumber women rates of violent crime, property crime and crime against women are higher

According to the UNHCR, 58 per cent of the more than 1 million migrants and refugees who arrived in Europe last year were adult men, 17 per cent were women, and the remainder were children.

Professor Valerie Hudson, a Texas A&M University professor, said gender imbalances could cause serious problems in the community.

She said societies where men outnumbered women were more susceptible to higher levels of violence, insurgence and mistreatment of women.

Sweden is of particular interest to Professor Hudson, because it has the second-highest number of asylum applications in Europe, relative to its population.

"I looked specifically at the figures that Sweden had provided on its migrants — 71 per cent of whom are male," she said.

"And then in the age group 16 to 17 years old, which is what I focused on, over 90 per cent are male."

Professor Hudson estimated there were about 123 boys for every 100 girls in Sweden.

In China, which only recently abolished its one-child policy, the ratio is about 117 boys for every 100 girls.

Professor Hudson said regardless of where the gender balance was the consequences were broadly the same.

"What we have found uniformly is a rise in crime, particularly violent crime, property crime, crime against women," she said.

"We also see much more ease of recruitment into criminal gangs, or quasi-rebel/social movement-type gangs.

"We also see across the board a more constrained lifestyle for women and there's also a significant rise in demand for trafficked women, either as 'brides' or prostitutes."

Gender imbalance 'may have been' contributing factor in Cologne

While Sweden is particularly transparent with its migration data, the top destination for asylum seekers fleeing Iraq, Afghanistan and Syria is Germany.

Professor Hudson said her colleague Andrea den Boer, from the University of Kent, was doing her own research on Germany's gender ratio.

"[Dr den Boer] found that the sex ratio alteration in the young adult population, according to her ... preliminary calculations [is] looking also to be abnormal at about 114 boys of that age for every 100 girls," she said.

Dr Hudson said problems stemming from gender imbalances could be developing in Germany, after women in Cologne were subjected to a series of sexual assaults on New Year's Eve.

"We don't know whether the attackers were migrants or not, there's conflicting reports, some say yes ... others say it's too soon to tell," she said.

"Nevertheless, from the reports that we have it does look like the overwhelmingly male nature of the migration wave, coupled with marginalisation among those migrants, may have been a contributing factor."

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Professor Hudson said it was staggering that European nations were so caught up in debating the impact of refugees' religion, but did not seem to consider maintaining a balanced gender ratio in the community.

"Speaking as an American who has studied global gender issues for a long, long time, all of us look to northern and north-western Europe as an example of cultures that have successfully pursued gender equality," she said.

"To now see folks like the Mayor of Cologne urging women to dress differently, to behave differently, that there may be certain 'no-go areas' for women, I think is tragic.

"It is utterly tragic, not just for these European countries, but for the rest of the world that has looked to these countries for so long in great admiration."

Australia has agreed to resettle 12,000 people displaced by the conflict in Syria and Iraq.

A spokesman for Immigration Minister Peter Dutton told the ABC so far 26 people had arrived in Australia as part of that humanitarian intake.

"Gender ratios are not a criterion the department considers when assessing visa applications," the spokesman said in a statement.

The spokesman said Australia would give priority to people it assessed as most vulnerable — "women, children and families with the least prospect of ever returning safely to their homes".