Although aging per se causes neither dementia nor Alzheimer’s disease, it is the most strongly associated risk factor for AD. Family history, or genetic predisposition, is another important risk factor; a history of AD in a first-degree relative (parent or sibling) increases the odds of developing AD three- to fourfold. A history of severe head injury that leads to brief loss of consciousness doubles the risk of developing AD. These three risk factors — age, genetic predisposition, and head trauma — meet the accepted epidemiological criteria for causal factors:

(1) they provide a plausible biological explanation

(2) their effects are strong and consistent. Other risk factors that have been investigated — such as maternal age, hypothyroidism, and exposure to environmentaltoxins such as aluminum or to chemicals such as benzene and toluene — have not been shown to meet the above criteria. Factors that apparently decrease a person’s risk for AD have also been identified. Among these, the most important appears to be educational and occupational attainment. People who achieve only a low level of education have double the risk of developing AD compared with those who have had 6 to 8 or more years of schooling. Education presumably increases the brain’s reserve capacity such that the clinical manifestations of AD are delayed or become more difficult to detect.

 Other factors that have been implicated as having a protective effect — but need to be confirmed by further and more careful studies — include postmenopausal estrogen replacement therapy, long-term use of antiinflammatory drugs, and cigarette smoking. As an increasing proportion of the population survives beyond the age of 85 years, more people will be at risk for developing a dementing disorder. Recent well-designed epidemiological studies have assessed the prevalence of all dementias (including Alzheimer’s disease) in diverse commu-nities around the world.

These surveys indicate that 25 to 35% of those 85 and over are affected by some form of dementia. In one such study conducted in East Boston the prevalence of AD alone in the 85-and-over age group was found to be 47%. The variations in prevalence rates are due primarily to differences in the criteria used by investigators to identify individuals with dementia and, more specifically, with AD. One of the major problems in all such community surveys of dementia is that 15 to 30% of the sample population may be unwilling, unable, or unavailable to participate.

Because it is possible that dementia might be more frequent among those who do not participate, the reported prevalence rates may actually underestimate the true prevalence. Regardless of these problems in methodology and the differences in the estimates, two impor-tant facts have emerged from the epidemiological studies conducted during the past several decades:

 • The prevalence of dementia increases in an exponential fashion with increasing age; that is, the percentage of the population affected doubles for every decade people live beyond the age of 65. Thus, if 10% of all people 65 and older have AD, 20% of the over-75 population will be affected and 40% of all those over age 85.

 • Since the Industrial Revolution, but particularly starting at the turn of the last century, life expectancy has been increasing. During the past 3 decades, improvements in public health measures, diet, and health behavior have brought about dramatic demographic changes, including a lower birthrate. Thus, today in most industrialized countries, the 85-and-older age group is the fastest-growing segment of population.

These two facts, the growing number of older people and the increasing incidence of dementia with age, point to an ever larger group of those at risk for Alzheimer’s disease.