Life history is a major source of adaptive individual and species differences: a critical commentary on Zietsch and Sidari (2020).

Zietsch and Sidari (2020) argue that life history theory does not offer an appropriate model for understanding inter-individual trait covariation. We examine in great detail four major claims of Zietsch and Sidari’s critique of life history theory as an explanation of inter-individual covariation among phenotypic, especially psychometric, traits. We review relevant theoretical and
empirical research overlooked by Zietsch and Sidari and conduct reanalyses of comparative cross-species “pace-of-life” and human behaviour-genetic data. Our results challenge their arguments concerning (a) the implications for life history theory of the absence of individual-differences-level covariation in a variety of species and (b) the genetic aetiology of psychometric life history covariance. None of the four major claims made by Zietsch and Sidari withstand close scrutiny. Critiques of life history theory, such as the one developed by Zietsch and Sidari, should (if they are to be more persuasive) show what alternative theory might be at least as successful as life history theory in consiliently and parsimoniously explaining patterns of covariation among various traits within and between species, including the predictable absence of such covariation among certain traits. Evolutionary life history theory—as applied to non-human animals and humans—offers one of the broadest and empirically best-supported frameworks for understanding individual differences in physiology, cognition, behaviour, reproduction, and other traits and life outcomes.