We tracked the relative integration and differentiation among life history traits over the period spanning AD 1800–1999 in the Britannic and Gallic biocultural groups. We found that Britannic populations tended toward greater strategic differentiation, while Gallic populations tended toward greater strategic integration. The dynamics of between-group competition between these two erstwhile rival biocultural groups were hypothesized as driving these processes. We constructed a latent factor that specifically sought to measure between-group competition and residualized it for the logarithmic effects of time. We found a significantly asymmetrical impact of between-group competition, where the between-group competition factor appeared to be driving the diachronic integration in Gallic populations but had no significantly corresponding influence on the parallel process of diachronic differentiation in Britannic populations. This suggests that the latter process was attributable to some alternative and unmeasured causes, such as the resource abundance consequent to territorial expansion rather than contraction.