Life history plasticity in humans: the predictive value of early cues depends on the temporal structure of the environment

Nettle et al. [1] explored the conditions under which it could be adaptive for humans to calibrate the development of life history traits (for example, reproductive timing) on the level of adversity experienced in early life. They concluded that external predictive adaptive responses (PARs)—in which early cues are employed to forecast the adult environment—can only evolve if environmental states show ‘almost perfect’ levels of year-to-year autocorrelation, in the order of 0.95 or greater. They suggested that annual autocorrelations could be empirically estimated from environmental time-series datasets; consistent findings of autocorrelations smaller than 0.95 would argue against accounts of human life-history plasticity that involve external PARs. The model by Nettle et al. is a welcome contribution to the human literature on life history plasticity and the authors should be commended for their emphasis on testability and quantification. Indeed, their model offers conceptual clarity and quantitative predictions in an area where vague verbal claims are the unfortunate norm. However, it is important to understand to what extent the present results may depend on specific assumptions in the model, and critically evaluate the plausibility of the latter. This is especially true for quantitative predictions (for example, the 0.95 threshold) that lend themselves to be employed as heuristics for the interpretation of empirical findings. Here, I argue that the model by Nettle et al. makes some unrealistic assumptions about the structure of environmental change and present a simple revised model in which past events can have both immediate and delayed effects on subsequent environmental states. I show that when delayed effects are incorporated in the model, the predictive value of early cues shows a notable increase and external PARs can evolve with annual autocorrelations in the order of 0.85 (or even 0.80) instead of 0.95. While the qualitative results obtained by Nettle et al. remain valid, the scope for the evolution of external PARs is larger than that implied in their paper. Future theoretical work should strive to clarify the temporal structure of human environments and systematically explore its implications for the evolution of life-history plasticity. Nettle et al. modelled environmental fluctuations with a first-order autoregressive process of the form