It is precisely because we do not know the fundamental (necessary) cause of anorexia nervosa that recourse has to be had to a multidimensional approach, faute de mieux. Although it has its limitations, a multidimensional approach permits one to consider a range of possible causal factors which not only act in an additive manner but may combine in a specific manner to bring about the illness: 'It is the interaction and timing of these phenomena in a given individual which are necessary for the person to become ill'.(32)
It is useful to provide a simple model of the way that two broad sets of factors may interact and augment each other (Fig,, 1). The outer circle represents the entire population in a developed 'Westernized' country. Within the circle there is a large sector representing females within an age range of 10 to 50 years who experience prevailing social pressures to acquire a slender body shape through dieting. Evidently only a small proportion of these women develop the illness. It is likely that for anorexia nervosa to develop it is also necessary to possess a genetic predisposition, represented by the small inner circle. The intersection of the inner circle and the large sector produces a small sector of females who have the genetic predisposition and also experience sociocultural pressures to lose weight, interacting to cause clinical anorexia nervosa.
Fig. 1 Diagrammatic illustration of the way that genetic predisposition interacts with sociocultural pressures to cause anorexia nervosa. There is no intention of drawing the different parts of the circle to scale as the relative sizes of the populations are not known.
Causal factors may not only interact as explained above, but they can also influence the content of the illness, its 'colouring', and its form. This modelling function is described by the term 'pathoplastic' which was introduced by Birnbaum.(33) Pathoplastic features are to be distinguished from the more fundamental causes of psychiatric illness, but they do exert a predisposing tendency as well as a modelling role.
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