Rensch's rule is an allometric law concerning the relationship between the extent of sexual size dimorphism (SSD) and which sex is larger. Across species within a lineage, size dimorphism will increase with increasing body size when the male is the larger sex, and decrease with increasing average body size when the female is the larger sex.

Examples of phylogenetic lineages that appear to follow this rule include primates, pinnipeds, and artiodactyls.

It was proposed by Bernhard Rensch

Bernhard Rensch

Bernhard Rensch was a German evolutionary biologist, and ornithologist who did field work in Indonesia and India. He is probably best known as one of the architects of the modern evolutionary synthesis, which he popularised in Germany...

in 1950.