Ancient medical writers and biologists elaborated different theories to explain the phenomenon of multiple births. The earliest extant texts are in the Hippocratic collection and in the physiological treatises of Aristotle. They express opposed ideas: for the Hippocratics multiple births are the result of an ideal conception, for Aristotle they are regarded as anomalies associated with notions of monstrosity and excess. These views shed light on ancient collective imagery. Three themes in particular are found in non-medical literature and iconography: twin birth as a model of ideal fecundity, the ambiguous status of twins of different sexes, and the relation of multiple births to monstrosity and animality, as evidenced by the motif of twins born from one egg.