TBI is common enough that the effects are large on a population-wide basis:
We found that the crude population contributions of TBI explained approximately...2%–6% of the population differences in the outcomes. The strongest population attributable risks were found for the severe outcomes, including psychiatric inpatient hospitalisation (PAF = 5.5%; 4.9%–6.1%), premature mortality (PAF = 4.7%; 2.9%–6.5%), and disability pension (PAF = 4.6%; 3.8%–5.3%).
The comment goes on to list statistics on various long-term effects of TBIs—worth reading, IMO.
Here is a series of comments by gwern with many citations of research into head trauma and brain injury.
This comment in particular cites a paper called…
The comment goes on to list statistics on various long-term effects of TBIs—worth reading, IMO.