Europe faces the threat of a rise in violence and sex attacks because of the overwhelming number of single male migrants arriving, an American professor has warned.

The vast number of teenage boys coming to the continent after fleeing poverty in Africa and the Middle East is creating imbalances in the number of men and women in some areas, it is claimed.

More than 66 per cent of adult migrants arriving through Italy and Greece in the past year were male, according to the International Organization of Migration – along with 90 per cent of unaccompanied under-18s.

Hundreds of people gather in front of Cologne's main railway station, where disorder broke out last week and groups of 'Arab and North African' men attacked dozens of women

Groups of revellers gather in the city centre during the celebrations last week which quickly turned to chaos

Valerie Hudson, a professor at Texas A&M University, said numerous academic studies have shown that ‘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,' she wrote in an article published on US website Politico. ‘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 behaviour due to marginalisation, 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 masculinised societies.’ Young men are thought to be more likely to leave countries such as Afghanistan, Iraq and Syria as they are at greater risk of being forced to join fighting groups or of being killed rather than just captured.

Figures compiled by Professor Hudson showed that among the general population of Sweden the ratio of 16 and 17-year-old boys to girls was now skewed at 123:100 because of the influx of migrants.

Swedish government statistics for 2015 up to November show that 71 per cent of all asylum applicants were male. Among unaccompanied minors there was a ratio of 11.3 boys for every girl.

A group of men set off fireworks during the New Year's Eve celebrations in Cologne last week. Police say groups of men (not pictured) were responsible for coordinated attacks on women

Some of the arrivals are expected to be joined later by female relatives, but they are unlikely to fully balance out the disparity.

The EU yesterday warned the flow of migrants coming into Europe was continuing at the same pace despite a landmark deal with Turkey. In November, EU leaders pledged three billion euros in aid for more than two million Syrian refugees in Turkey, in exchange for it acting to stem the flow.

But the number of migrants travelling in the last couple of weeks via Turkey remains ‘relatively high’, European Commission vice president Frans Timmermans said at the launch of the Netherlands’ six-month EU presidency.