We constructed a simple model to show that the variability of nest structures in the social wasp genus Polistes could result from a simple algorithm modulated by a single parameter. The interplay between the simple algorithm and the geometry of the nest structure resulted in isotropic growth. New construction on the sides of the comb was initiated according to the size of the sides. Due to the geometry of the comb a new rule emerged: if one side of the structure is extended, it decreases the chance that that side will be further extended in the next step (because the number of possible initiation positions decreases), but at the same time construction on one side increases the probability of the extension of that side's neighbours (because their possible initiation positions increase). This emergent isotropic growth keeps the nest almost circular, regardless of nest size, and it ensures well–centred nests. Eccentric nests reflect differences in initiation preference towards particular sides of the previously built structure. Interplay between structural constraints and the simple behavioural algorithm, which is governed by both structural and ‘external’ information, resulted in nests of different shapes. The model was a good predictor of the range of Polistes nest forms found in nature.