The demonstration that resilient and vulnerable responses to stress are dramatically distinct in populations of genetically identical mice with highly controlled environmental histories41 (see Supplementary information S1 (box)) raises the possibility that a third type of mechanism — epigenetics — could be involved. The hypothesis is that stochastic, epigenetic changes that occur during brain development are an additional means by which behavioural variability is generated in individuals, better preparing the species for a host of possible environmental challenges. According to this scheme, random epigenetic changes that drive resilience would promote survival during periods of extreme duress, whereas those associated with vulnerability would generate animals that cope better in times of plenty.
