He who fights and runs away . . .: withdrawal, aggressive defence and numbing

We have seen that even unicellular organisms may learn withdrawal or flight responses and that predation pressure may have driven the early development of multicellularity, a process taking over 1000 million years – more than half the duration of life on earth (Stearns and Hoekstra 2000). Withdrawal had evolved further by the reptilian era, albeit reptiles did not have the flight capacity of mammals. We have established that withdrawal is under a different selection regime from avoidance (Brodie et al. 1991). A PTSD sufferer may avoid supermarkets. If pressured to override such avoidance, approach is often followed by flight. Hence, in clinical practice avoidance is the more common complaint. This mirrors the costbenefit analyses discussed earlier (Vermeij 1982; Lima and Dill 1990; Lima 1998), whereby avoidance is usually less costly in terms of energy expenditure (and embarrassment) than flight. PTSD sufferers invited to confront certain of their avoidance behaviours often respond, ‘It’s not worth the hassle’, a cost-benefit statement.