Study: Rates of Dementia are Going Down

For more than a century, people, pundits, politicians, and scientists have debated whether success, health, and wealth are the result of genetic endowment (Nature: “They were born that way”) or environment/choice (Nurture: “They were raised that way” or “They chose to…”).

Scientific advances over the past half century have resolved this contentious dilemma. Most of the time, the truth lies somewhere in the middle. These discoveries have moved us beyond simplistic models (“exposure to X in childhood causes Y”) and superficial solutions that are doomed to fail. While the ultimate answers will be complicated, they will be more reflective of the complex nature of many common problems.

Take alcoholism as an example. It has long been recognized that alcoholism runs in families, a finding compatible with both a genetic explanation (it was inherited from a parent) and an environmental one (it was induced by exposure to a drinking culture during youth). One way to distinguish between these two explanations is to study identical and fraternal (non-identical) twins. A variation of this approach is to compare those adopted out at birth, some of whom were born to families in which there was a history of alcoholism and others who were not, and some of whom were adopted by families in which there was alcoholism and others in which there was not. The results of a number of such studies show the following:

• The risk of becoming a heavy alcohol user is greater in identical twins than fraternal twins (supporting a genetic contribution);
• Twins born to parents who had alcoholism and adopted by parents with alcoholism had the highest risk of themselves having alcoholism;
• Those born to and adopted by families without alcoholism had the lowest risk; and
• Those born to a parent with alcoholism and adopted by a family without alcoholism as well as those born to a family without alcoholism and raised in a family in which there was alcoholism had an intermediate risk (supporting an environmental and genetic contribution).
• Overall, these studies suggest that the heritability of alcoholism is about 50%, meaning that about half of the risk of developing an alcohol use disorder is genetic and that the environmental contribution is in a similar range.

The sequencing of the human genome in 2001 has led to other studies of the Nature/Nurture question in numerous medical and psychological disorders. The results support the conclusion that many common conditions are both genetically and environmentally influenced. The operative word here is “influenced.” In many disorders both genes and environments act by increasing or decreasing risk, not absolutely determining the outcome.

I find this both liberating and positive because it means that neither genes nor environment determine our destiny regarding many of the physical, medical, and social ills of our times. It also means that we should be able to identify environmental modifiers of negative genetic attributes and increase the likelihood of beneficial, positive outcomes.

This approach may already be coming to fruition in dementia. Three recent studies from Framingham, Massachusetts (available here), Rochester, Minnesota, and Rotterdam, The Netherlands have found that the incidence of dementia, that is, the rate of new cases per year, is decreasing. Why? It’s most likely better control of mid-life high blood pressure, treatment of elevated cholesterol and abnormal blood lipids, and increased exercise. Since blood pressure and cholesterol are both genetically controlled and environmentally influenced, this hopeful finding is a wonderful example of how changing ‘environment’ can modify the genetic risks of Alzheimer’s disease.

This is a complex challenge. Since many conditions are influenced by tens or hundreds of genes and multiple environmental influences, the process of disentangling these complex webs will take time.

So, while the answer to the Nature/Nurture debate turns out to be complex, it offers a way forward that can improve life in general and resoundingly validates the efforts to bring wellness to the population.

Dr. Rabins is the former Richman Family Professor of Alzheimer’s and Related Diseases in the Department of Psychiatry and Behavioral Sciences at John Hopkins University, and is currently Professor at the Erickson School, University of Maryland, Baltimore County. He is the co-author of ‘The 36-Hour Day’, a book about caring for patients with dementia, which is in its fifth printing. This article was adapted from Welltower’s blog Aging Insights.