Specifying the timescale of early life unpredictability helps explain the development of internalising and externalising behaviours and their link with reproductive effort in adolescence

Early life unpredictability associated with both physical and mental health outcomes throughout the life course. Here, we classified adverse experiences based on the timescale on which they are likely to introduce variability in children’s environments: variations unfolding over short time scales (e.g., hours, days, weeks) and labelled Stochasticity vs variations unfolding over longer time scales (e.g., months, years) and labelled Volatility and explored how they contribute to the development of problem behaviours. Results indicate that externalising behaviours at age 9 and 15 were better accounted for by models that separated stochasticity and volatility measured at ages 3 to 5. Whereas internalising behaviours were not associated with any form of unpredictability, externalising behaviours were specifically associated with volatility. This positive association was mediated by greater reproductive efforts. These findings are interpreted in light of life history models of psychopathology and reinforcement learning models of learning under uncertainty.