When small effect sizes become huge: Synaesthesia is linked to very large differences in cognition.

The replication crisis has taught us to expect small-to-medium effects in psychological research. But this is based on effect sizes calculated over single variables. Mahalanobis D , the multivariate equivalent of Cohen's d , can enable very large group differences to emerge from a collection of small-to-medium effects (here, reanalysing multivariate datasets from synaesthetes and controls). The use of multivariate effect sizes is not a slight of hand but may instead be a truer reflection of the degree of psychological differences between people that has been largely underappreciated.