Rees Chapman, Ph.D.
July 2012

Dementia is a deterioration of mental function that affects memory, cognition, language, higher order reasoning, socioemotional function and behavior.  It ranks second among health concerns for the elderly, after cancer.  Common types of dementia are Alzheimer's, vascular, frontotemporal, semantic, and Lewy body, but there are thought to be fifty or more different causes and associated forms of the disorder.

The declining mental function attributed to dementia is usually rather easy to detect in a patient, particularly in its later stages, and this results in many older individuals being declared demented by laypersons such as family members, as well as non-specialists such as family practice physicians.  Unfortunately, there are many other mental, medical, and social disorders and diseases that resemble dementia, and can lead to erroneous diagnoses and neglect of the actual causes of the dysfunction.  These can include:
A particularly good review of the issues of differential diagnosis may be found at

I would never make other than a provisional diagnosis of dementia for a patient in the absence of a thorough medical evaluation.  It troubles me that a large proportion of patients placed in nursing homes and congregate facilities have never been evaluated neuropsychologically, in which empirical data and clinical observations are obtained and interpreted against comprehensive accounts of their histories and qualities of function.  As a result, a great many elderly people whose forgetfulness and withdrawal are better attributed to depression or some other disorder are said to be demented by their primary care physicians, often relying heavily upon the descriptions of family members.  Patients' loss of autonomy (which could be effectively treated) is hastened and worsened by misdiagnosis and improper intervention.  Then again, I have occasionally been asked to evaluate patients who have many signs and symptoms of dementia but have not been thoroughly assessed medically, and they are subsequently found to have nutritional or vitamin deficiencies attributable to unhealthy diets resulting from financial inadequacies.  Thus, I believe it is problematic and irresponsible to conclude dementia in patients without comprehensive multidisciplinary evaluations by professionals ruling out a multitude of other possible causes that may be resolved by appropriate treatment.

Over the years, I have arrived at a somewhat unique perspective on the dementing process that views it in a context of lifespan processes and sociobiological factors.  There can be little doubt that the average human now lives many more years than his or her historical predecessors. Medical and social advances have increased the human life expectancy remarkably, from about 20 in the Neolithic eras to late 20s in Greek and Roman times to early 30s in the Middle Ages to 67 in 2010.  Arguments are made that many other factors (e.g., infant mortality, plague, warfare) complicate the estimates of life expectancy, and many historians wisely differentiate this from longevity.  But the nearly undeniable truth is that people generally didn't live as long thousands of years ago as they do today.  In terms of evolutionary process, we human organisms are not substantially different today from biblical times; our average brain size today (1100 to 1500 cubic cm) is little different from that of Neanderthals at 1200 to 1900 cubic cm), albeit slightly smaller, and while brain size is not an absolute predictor of neurological complexity or intelligence, it can be argued that the anatomical design of our brains is minimally changed over the last 2000 years.  Processes of natural selection lead populations of organisms to live more successfully with certain features and qualities that give them competitive advantages, and those characteristics are generally passed on to subsequent generations.  Thus, we can assume that the brain capacities of humans in Roman times were adequate for the average person to live to about age 30; while a bigger brain might have provided certain advantages, it would also come with certain liabilities and associated weaknesses.  So the average Roman had a brain with an adequate capacity for processing and storing information for about three decades of life.  But remember: our brains' design and capacity are essentially the same as those of our ancestors two millennia ago.  Which means that as we approach age 40, we begin to exhaust the available neural space to store new information, having retained as much as we needed to live and reproduce successfully two thousand years ago.  Evidence for this may include research that shows a significant reduction in mental capacity (such as memory, processing speed) after age thirty.  And there's another factor: the sheer complexity of information we store in our brains today.  Compared with the days of Rome, even the Middle Ages, the quantity and levels of abstraction of the data we process and retain are vast.  This is likely to hasten the exhaustion of available mental space even more. 

Thus, our brains, evolved with storage capacities for limited quantities and complexities of information, may actually run out of space as we enter middle age.  To the extent this may be true, the dementing process may be, to some degree, a normal lifespan phenomenon that manifests differently in each person, and that would be more pathological in some.  I must point out that this is but speculative theorizing, and includes hypotheses which probably cannot be tested empirically.  But I hope it offers a useful perspective on the causes of dementia.
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