Between 7% and 33% of patients who have “mild” traumatic brain injury (sometimes called concussion) develop persistent post-concussion syndrome, which may last weeks to months after injury.1 More than 15% have a measurable cognitive deficit at one year.2 3 There is growing interest in the syndrome of post-traumatic encephalopathy,4 5 which may follow a blast injury or repeated sports related concussion. However, despite this growing literature on the cognitive consequences of mild traumatic brain injury, our knowledge of risk factors that predispose people to sustaining such injury is limited.

In a linked paper (doi:10.1136/bmj.f723), Nordström and colleagues examine the associations and temporal associations between a history of concussion, cognitive function, academic achievement, and measures of social wellbeing in a cohort of more than 300 000 Swedish conscripts.6 Given the paucity of data on premorbid neurocognitive testing in traumatic brain injury, this paper draws on an impressively large dataset that allows comparison of …