1. Sexual size dimorphism (SSD) is a common phenomenon in animals. Rensch's rule states that larger species generally exhibit a higher male to female body size ratio than smaller ones. 2. Domesticated animals offer excellent opportunities for testing predictions of the functional explanations of Rensch's rule and this was tested in a meta-analysis of SSD in 38 breeds of domestic geese compared among themselves and with their wild relatives (subfamily Anserinae, 35 species). 3. Domestic geese and wild Anser species taken together supported Rench's rule but the wild species did not. 4. The non-targeted sex selection hypothesis seems to provide the best intuitive explanation for the lack of SSD in geese.