It shows that half of all homeless people may have suffered a TBI at some point in their life – either as a consequence or even the cause of their homelessness.

A large study compiling research results from six high-income countries – Australia, Canada, Japan, South Korea, the UK and the US – found that 53 per cent of homeless people had suffered a TBI. This is estimated by the researchers to be 2.3 to four times the rate for the population as a whole.

One in four of the injuries was moderate to severe, according to the study published in the Lancet.

Jehannine Austin, of the British Columbia mental health and substance use services research institute in Canada, where the
study was carried out, said: “The relationship between homelessness and TBI could function both ways – TBI could increase the risk of homelessness, and homelessness could increase the risk of TBI.

“We need a better understanding of this relationship to address the issue, and to improve outcomes in the homeless and marginally housed population.”

Jesse T Young, a research fellow at the University of Melbourne, Australia, said: “It is becoming clear that TBI can be both a cause and a consequence of homelessness. The functional and socioeconomic consequences associated with TBI can present challenges to finding and retaining housing.”

These latest findings are in line with previous studies which suggest close links between life on the streets and a heightened risk of TBI.

Studies in the UK and North America over the last decade have found levels of past TBI experiences among homeless people to generally range between around 45 and 55 per cent. Research also suggests the vast majority of TBIs happened before homelessness occurred.

A 2012 study by the Disabilities Trust, for example, found that 48 of the 100 homeless people it questioned in Leeds had experienced a head injury.

Of those, 90 per cent suffered the injury before they became homeless.

The 48 per cent figure was more than double the proportion of head injuries reported by those in a comparator control group of non- homeless people.

Another Disabilities Trust study, in Glasgow, used hospital records of admissions to assess the city’s homeless population.

It found that the frequency of admission to hospital with head injury among the homeless was five times higher than that of the city’s general population.

In Toronto in 2004/05, 601 men and 303 women at homeless shelters and meal programmes were surveyed.

Overall, 53 per cent had experienced a TBI, with 12 per cent reporting a moderate or severe TBI. In this study 70 per cent of respondents sustained their injury before they were homeless.

In 2014, researchers at St Michael’s Hospital in Toronto surveyed 111 homeless men and found that 45 per cent of them had suffered at least one TBI in their life, and 87 per cent of those injuries occurred before they were homeless.

Among the general population, TBI rates are estimated to be 12 per cent, according to a 2013 meta-analysis of studies from
developed countries.