Despite the undoubted usefulness of syndromic approaches to mental disorders, from which the above is derived, there is concern with them. This is partly because they tend to see symptom clusters as disorders and distinct categories that can overlap with other disorders, and vary in levels of severity, but are distinct nonetheless (Gilbert, 1992). It is clear, however, that depression is a highly heterogeneous disorder with a variety of proposed subtypes, such as neurotic-psychotic, bipolar-unipolar, endogenous-reactive, primary-secondary, early onset versus late onset, angry versus anxious, agitated-retarded, serotonion versus noradrenalin based, and various mixed states (Gilbert, 1984, 1992), with a new category of atypical depression also being suggested (Posternak & Zimmerman, 2002). In fact, depression is more often than not comorbid with other (especially anxiety) disorders (Brown et al., 2001). To complicate the picture further, Akiskal and Pinto (1999) suggest that a substantial minority of depressions are related to a spectrum of bipolar disorders, some of which may be destabilised on traditional antidepressants. Coyne (1994) raised major concerns about dimensional approaches (for example, mild, moderate, and severe), and, assuming that results from studying mildly depressed or dysphoric people (for example, some students) can be extrapolated to more severe depression, as there may be quite different process involved.
Coyne's view has been challenged (e.g., Vredenburg et al., 1993), but this debate on types of depression and spectrums of disorder, and variations in physiological, psychological, and social processes, raises a key issue about the models we use to investigate processes. We can consider the question of discontinuities in severity of depression (that is, some people have more severe depressions than others) and whether discontinuities necessarily suggest different processes (such as causes and vulnerability factors) and/or different relationships (interactions) between processes. For example, catastrophe theory (Zeeman, 1977), later to evolve into chaos theory, points out that processes that are themselves dimensional and linear can produce discontinuous effects according to the state of the system. Many systems are like this in fact; for example, the straw that breaks the camel's back, the wave breaking on the beach, or the animal that is fighting and then suddenly, as fear gets the better of it, turns tail and runs away. And in anxiety disorders we talk of 'panic attacks' as a sudden onset of a major change in the system. These, then, are points of sudden shifts and discontinuities. Hence, in 1984, I argued that taking a dimensional approach to depression is not to assume linearity (that is, a bit more of this causes a bit more of that). I used what is called the 'cusp catastrophe' (Zeeman, 1977) to explore this (Gilbert, 1984, pp. 199-215). Hence, although there are different types of depression with different severities, there may still be similarities in the processes and types of stressor that trigger them. However, due to system organisation factors (such as genes or early trauma), small variations in one dimension (for example, a rejection that makes one feel unloved) can produce catastrophic effects in another part of the system (such as stress hormones). This then causes the system to spiral down or dramatically shift to a new equilibrium (catastrophe shift) way below what (say) someone else might experience. We often call these pre-depression factors 'vulnerability factors', but another way to think of them is as 'system setters'; they set a system up in a certain way and can produce catastrophic effects in some contexts. We will return to this concept as we proceed through the chapter. More recently, Meehl (1995) has offered a fascinating and important discussion of these difficulties in classification (for example, categorical versus dimensional) and calls for new mathematical models for their study.
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