“What we found was that … you only really need to have three lifetime concussions to have some kind of cognitive deficits in the long term,” said Dr Matthew Lennon, the study’s lead author and a PhD candidate at the University of New South Wales’s Centre for Healthy Brain Ageing.

A large and growing body of scientific evidence has shown links between repeated exposure to head injury and sub-concussive blows in contact sports and the neurodegenerative disease chronic traumatic encephalopathy (CTE), which has been found in the brains of multiple Australian sportspeople, from amateurs to professionals.

Lennon’s research found that while people who had experienced repeated concussions had measurably worse cognitive performance, the differences were not drastic.

The paper argued, however, that given the “hotly debated” issue of when people ought to stop participating in higher-risk activities, such as contact sports, the finding that three or more concussions caused long-term cognitive deficit offered a benchmark.

The research, published in the Journal of Neurotrauma, forms part of a wider project known as the Protect study, which follows UK participants for up to 25 years to understand factors affecting brain health in later life.

“The retrospective design of the study, with elderly participants often recalling details of events more than three decades in the past, may have caused an underreporting of head injuries and thus an underestimate of the size of their effect,” they wrote.