It's been widely reported that football and other contact sports increase the risk of a debilitating neurological condition called chronic traumatic encephalopathy (CTE).
CTE is believed to be the cause of behavioral symptoms including irritability, anger, aggression, depression and suicidality; and cognitive symptoms including impaired learning, memory, language, information-processing speed and executive functioning.
CTE is said to be linked to concussions and characterized by the buildup of abnormal substances in the brain called tau proteins.
But so far there is only limited evidence to support this CTE theory, the researchers write.
"Older persons without dementia can accumulate Alzheimer's disease pathology (including tau deposition) without any associated cognitive or clinical symptoms," the researchers say.
"The actual clinical significance of 'abnormal' tau deposition in the brains of retired athletes therefore remains unclear."
"One cannot deny that boxing and other contact sports can potentially result in some type of injury to the brain," Karantzoulis and Randolph write.
"There currently are no carefully controlled data, however, to indicate a definitive association between sport-related concussion and increased risk for late-life cognitive and neuropsychiatric impairment of any form."