Theoretical PaperAn approach to the prevention of anxiety-related disorders based on evolutionary medicine
Introduction
Fear is an innate mammalian function with an obvious adaptive value [1], [2], [3]. Compared to other complex functions involving the brain, fear has a reasonably well-understood neurology, endocrinology, and genetics [4], [5], [6], [7]. Fear is an effective stimulator of the ‘fight-or-flight’ response, also referred to as the acute stress response, which implies activation of the hypothalamic–pituitary–adrenal (HPA) axis. The rational for this response is presumably to prompt the animal to either flee (if escape is likely) or freeze/hide (if escape is not an option).
The term anxiety may be considered synonymous with fear, but is typically used for the debilitating state caused by inappropriate or excessive triggering of the fear function, such as in phobias and generalized anxiety disorder. Long-term excessive or inappropriate triggering of the fear function may be defined as anxiety disorders according to diagnostic manuals (ICD/WHO or DSM/APA). In addition to reducing the quality of life of the sufferer [8], the condition may lead to various secondary ailments, such as stress-related diseases [9], [10] and depression [11]. Moreover, in both animals and humans, a high level of anxiety correlates with a shortened life span [9], [12], [13].
According to the National Institute of Mental Health, some 10% of adult Americans suffer from anxiety disorders [14]. Considering the secondary consequences of excessive anxiety and stress, and that even milder versions of anxiety, not considered pathological, may reduce quality of life [8], anxiety appears to be among the more taxing health problems in modern societies [15].
The evolutionary perspective has proven valuable in suggesting causative factors where the prevalence of a disease or a condition is in excess of what one might reasonably expect would be the case for humans living in an environment to which they were genetically adapted [16], [17], [18], [19]. The high proportion of people suffering from debilitating problems related to anxiety indicates that this is such a condition; in a natural setting, one would not expect such a function to become incapacitating except in extreme cases. I shall combine current understanding of the fear function with the evolutionary perspective in order to propose possible causes for the prevalence of anxiety-related problems, and thereby point towards preventive measures.
A central assumption of evolutionary medicine is that, as a rule of thumb, both animals and humans are designed to thrive under the conditions to which their genes are adapted. These conditions, sometimes referred to as the environment of evolutionary adaptation are, in the case of humans, presumably akin to the typical way of life of the late Pleistocene, prior to the invention of agriculture [18]. Disparities between this way of life and that of present societies are referred to as mismatches. However, not all mismatches are unfavorable, for example, sleeping on a mattress may be as healthy as sleeping on the ground. The mismatches that are potentially detrimental may be referred to as discords [20].
Section snippets
Fear in an evolutionary perspective
There are differences as to what sort of signals animals are predisposed to respond with fear, and as to how fear is learned, but the underlying neurology and endocrinology of the response appear to be remarkably similar across mammalian species [4], [6]. Thus, animal models are presumably useful for understanding the physiology of fear. Moreover, some of the health consequences of excessive anxiety and stress also emerge in mammals.
The importance of reacting with fear in a situation that may
The hyperactive fear function
It is generally recognized that most bodily and mental function will tend to increase in power and performance upon stimulation and use. The fear function appears to be no exception. The expansion of the endocrine glands involved in stress has been known for a long time [27]. People prone to respond with fear tend to develop a more pronounced HPA response than do less fearful individuals [4], [6]. Moreover, it is well known that brain functions tend to develop upon use, by both physical
Discords that may explain excessive fear function activity
Although we know that some genotypes are more vulnerable than others [23], [24], genetic epidemiology suggests that anxiety-related problems are also due to environmental conditions, including social interactions [2], [33]. Particular traumatic experiences have been assumed to be the environmental factor causing phobias, for example, nearly drowning at a young age might cause adult water phobia. However, recent research suggests that other environmental factors may be equally important [21],
Research that may substantiate proposed discords
To the extent that the problem of anxiety is due to discords in the modern environment, it is important to conduct research to find these discords and estimate their relative importance. The above suggested discords may be evaluated by epidemiological methods, either by comparing cultures with different practices or by correlating anxiety with various childhood experiences within a culture. However, confounding factors are an important problem of cross-cultural comparison, while the homogeneity
Conclusion
Although fear is an important innate function, one would expect that in the absence of causative environmental factors, fear should only rarely develop into a pathological and disabling feature. I suggest the following evolutionary medicine-based model for explaining the present pervasiveness of anxiety-related problems:
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The fear function is designed to be easily activated, which makes it particularly vulnerable to discords, in the form of excessive or abnormal activation, and to subsequent
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