Assessing causal pathways between physical formidability and aggression in human males

Studies suggest the existence of an association between the physical formidability of human males and their level of aggression. This association is theoretically predictable from animal models of conflict behavior but could emerge from multiple different causal pathways. Previous studies have not been able to tease apart these paths, as they have almost exclusively relied on bivariate correlations and cross-sectional data. Here, we apply longitudinal twin data from two different samples to (1) estimate the direction of causality between formidability and aggression by means of quasi-experimental methods and (2) estimate the relative contribution of genetic and environmental factors by means of twin modeling. Importantly, the results suggest, on the one hand, that the association between formidability and aggression is less reliable than previously thought. On the other hand, the results also suggest that when the association occurs, the causal direction is from formidability to aggression and the primary part of the causal relationship is genetic in nature. These latter findings are consistent with adaptionist models suggesting that human male aggression is ‘reactively heritable’ to genetic components of formidability.