The Field Glass (1963)
by Rene Magritte
Almost everyone dissociates to some degree, as this illusion illustrates. Dissociation is a fairly normal coping strategy in the face of overwhelming stress, but extreme dissociative tendencies may be pathological. At this extreme, Dissociative Identity Disorder, or DID (formerly called MPD), is a condition requiring specialized treatment. Using Taxometric Analysis, Niels Waller and colleagues identified a separate ‘taxon’ of pathological dissociation useful in screening suspected dissociative disorders. This clinicially-important probability score is calculated from a subset of items in the DES, or Dissociative Experiences Scale. Thankfully, Darryl Perry has translated this algorithm into a downloadable DES Taxon Calculator for (for MS-Excel), described on and available from ISSTD‘s website.
As you might expect, risk for PTSD increases with exposure to trauma. In other words, chronic or multiple traumatic experiences are likely to be more difficult to overcome than most single instances. PTSD is also more likely if passive defenses, such as freezing or dissociation, are used — rather than active defenses such as fight or flight. Epidemiological estimates suggest that the incidence and lifetime prevalence rates of PTSD in the general population are around 1% and 9%, respectively. But these levels increase markedly for young adults living in inner cities (23%), and for wounded combat veterans (20%). There is also evidence that early traumatic experiences (e.g., during childhood), especially if these are prolonged or repeated, may increase the risk of developing PTSD after traumatic exposure as an adult. This may result from state-dependent learning, where previous responses to a terrifying event are repeated even though more appropriate responses (i.e., active defenses) may now be possible.
Several animal studies have suggested the possibility of permanent physical damage (including shrinkage) in the hippocampus and changes in the amygdala when severe or chronic trauma — and its symptoms — persists (see especially work by Robert Sapolsky and by Joseph LeDoux, respectively). Unfortunately, there is no easy way to compare the relative types or degree of trauma across species. Human data, including Gilbertson et al’s (2002) twin study, suggest that response to trauma may be influenced by pre-existing individual differences in hippocampal volume. Perhaps both processes are involved.
There’s no clear evidence that susceptibility to PTSD varies for members of different ethnic or minority groups (given a traumatic experience). But individual differences clearly play some role. For example, younger children have less ability to predict, avoid, make sense of, or to actively defend against, upsetting events, and more introverted or shy individuals may experience stronger emotional reactions to such experiences.