Saga literature abounds with detailed physical descriptions of individuals which help define the personalities of the characters described. While many descriptions derive from the Latin physiognomic tradition, others do not fit well into this tradition. This study applies recent scientific research to show a relationship between physical portraits and behaviour in the sagas which may have an ultimately biological origin. There is a large body of research which shows that the hormone testosterone affects behaviour as well as physical development. Some studies have shown a statistical relationship between behaviour and certain physical features related to testosterone. For this study, I identify terminology which corresponds to various types of testosterone-related physiology and the characters who are described with these terms. I then assess the behaviour of individuals showing these characteristics by identifying descriptions which correspond to aggression and dominance, or peacefulness and deference. Individuals who are portrayed as having physiology consistent with high testosterone (such as broad facial features, thick facial hair, and broad shoulders) are also portrayed with testosterone-related behaviours, particularly dominant and aggressive behaviour. This study posits a native physiognomy based on observations of physical features and aggressive behaviour which forms the basis of the descriptions in the sagas.