those with fewer years of formal education.77,152-156 The underlying reasons for the relationship between formal education and reduced Alzheimer’s risk are unclear. Some 16 Alzheimer’s Association. 2022 Alzheimer’s Disease Facts and Figures. Alzheimers Dement 2022;18. researchers believe that having more years of education builds “cognitive reserve.” Cognitive reserve refers to the brain’s ability to make flexible and efficient use of cognitive networks (networks of neuron-to-neuron connections) to enable a person to continue to carry out cognitive tasks despite brain changes.157-158 The number of years of formal education is not the only determinant of cognitive reserve. Having a mentally stimulating job and engaging in other mentally stimulating activities may also help build cognitive reserve.159-162 Other researchers emphasize the indirect effects of formal education, such as its effects on dementia risk through socioeconomic status. Having fewer years of formal education is associated with lower socioeconomic status.163 This in turn may increase one’s likelihood of experiencing poor nutrition; decrease one’s ability to afford health care or medical treatments, such as treatments for cardiovascular disease risk factors that are closely linked to brain health; decrease one’s ability to engage in educational opportunities outside of formal schooling; and limit one’s access to physically safe housing and employment. The latter could increase one’s risk of being exposed to substances that are toxic to the nervous system such as air pollution,164 lead165 and pesticides.166 In addition, people with fewer years of education tend to have more cardiovascular risk factors that lead to cerebrovascular disease, including being less physically active,167 having a higher risk of diabetes,168-170 and being more likely to have hypertension171 and to smoke.172 Social and Cognitive Engagement Additional studies suggest that remaining socially and mentally active throughout life may support brain health and possibly reduce the risk of Alzheimer’s and other dementias.124,173-185 Socially and cognitively stimulating activity might help build cognitive reserve. However, it is also possible that undetected cognitive impairment decreases one’s interest in and ability to participate in activities involving social and cognitive skills. In this case, the association may reflect the effect of cognitive impairment on social and cognitive engagement rather than the effect of engagement on dementia risk.186 More research is needed to better understand the biological processes that link social and cognitive engagement to dementia risk. Traumatic Brain Injury (TBI) TBI is a head injury caused by an external force to the head or body resulting in disruption of normal brain function.187 TBI is associated with an increased risk of dementia.188-189 According to the Centers for Disease Control and Prevention (CDC), nearly 3 million TBI-related emergency department visits, hospitalizations and deaths occurred in 2014, the latest year for which this information is available.187 In 2017, there were nearly 224,000 TBI-related hospitalizations. The leading causes were falls (49%) and motor vehicle crashes (24.5%).187 Two ways to classify the severity of TBI are by the duration of loss of consciousness or post-traumatic amnesia190 and by the individual’s initial score on the 15-point Glasgow Coma Scale.191 • Mild TBI (also known as a concussion) is characterized by loss of consciousness or post-traumatic amnesia lasting 30 minutes or less, or an initial Glasgow score of 13 to 15; about 75% of TBIs are mild.192 • Moderate TBI is characterized by loss of consciousness or post-traumatic amnesia lasting more than 30 minutes but less than 24 hours, or an initial Glasgow score of 9 to 12. • Severe TBI is characterized by loss of consciousness or post-traumatic amnesia lasting 24 hours or more, or an initial Glasgow score of 8 or less. The risk of dementia increases with the number of TBIs sustained.188 Even those who experience mild TBI are at increased risk of dementia compared with those who have not had a TBI. A study found that mild TBI is associated with a two-fold increase in the risk of dementia diagnosis.193 Studies have also found that people with a history of TBI who develop Alzheimer’s do so at a younger age than those without a history of TBI.194-195 Whether TBI causes Alzheimer’s disease, other conditions that lead to dementia, or both, is still being investigated. The relationship between TBI and chronic traumatic encephalopathy (CTE) is a growing area of research. CTE is a neuropathologic diagnosis (meaning it is characterized by brain changes that can only be identified at autopsy) associated with repeated blows to the head, such as those that may occur while playing contact sports. Among former amateur and professional football players, the odds of developing CTE, which is associated with dementia, increased 30% per year played.196 Currently, there is no test to determine if someone has CTE-related brain changes during life. A review article indicated that the greatest risk factor for developing CTE-related brain changes is repetitive brain trauma — repeated, forceful blows to the head that do not, individually, result in symptoms.197 Like Alzheimer’s disease, CTE is characterized by tangles of an abnormal form of the protein tau in the brain. Unlike Alzheimer’s, beta-