What Type Of Stress And What Type Of Defence Some Evolved Regulators

Although we can be stressed by many different life events and loss of control, from an evolutionary point of view we need to identify stressors that can be chronic and that down-regulate PA. Depression (low PA) makes us withdraw effort from things that are unlikely to work out—this is called disengagement (Klinger, 1975)—or where to continue to engage in activities increases the risk of harm (Nesse, 2000). However, there are at least two meaningful evolutionary contexts where downregulation of PA may have been adaptive: attachment loss and entrapped defeat. This is not a new idea but has been suggested by many researchers (e.g., Beck, 1987; Sloman, 2000a). There are two central concepts to understanding how these work. First, in earlier evolutionary times, in certain contexts, the signals an animal emits may themselves be threats. This is because the animal may behave in a way that draws unwanted attention to itself (as from a predator or hostile conspecifics). Hence, an adaptive defensive response would be to stop the animal from behaving (emitting signals) in certain ways and thus reduce the risk of harm. The second point is that many defensive responses are designed for rapid action, and to be accessed quickly and produce discontinuities in behaviour; for example, when an animal needs to stop feeding and run from a predator, or to shift from one defence to another—for example, from fight to flight (Gilbert, 1984). Hence, a shutting down of 'signal emission' and a rapid shift in behaviour may be part of an evolved design for both attachment loss and defeats—a theme we will explore below.

The idea that some defensive responses are designed to stop an animal from engaging with its environment (Gilbert, 1989) and from seeking resources fits with the idea that the primary PA system that will be downregulated for this behaviour is the anticipatory PA system (Davidson, 2000). Moreover, studies of nonverbal behaviour in depression by Schelde (1998a;b) suggest that depression is marked by a reduction in signals for social engagements (as in eye gaze, facial expression, and speech), a feature which changes with recovery. Ellgring (1989), however, notes that there are sizeable individual differences in the non-verbal behaviour of depressed people, and much depends on what other affects are part of the clinical picture (for example, some patients are more anxious, and others more angry).

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