A key challenge for social psychology is to identify unifying principles that account for the complex dynamics of social behaviour. We propose psychological relativity and its core mechanism of comparison as one such unifying principle. To support our proposal, we review recent evidence investigating basic processes underlying and novel applications of social comparisons. Specifically, we clarify determinants of assimilation and contrast, evaluative consequences of comparing similarities vs. differences, attitudinal effects of spatial relativity, and how spatial arrangements determine perceived similarity, one of the antecedents of social comparisons. We then move to behavioural relativity effects on motivation and self-regulation, as well as imitation behaviour. Finally, we address relativity within the more applied areas of morality and political psychology. The reviewed research thereby illustrates how unifying principles of social cognition may be instrumental in answering old questions and discovering new phenomena and explanations.