Muscular active types (mesomorphs), the theory went, tend to be more aggressive and physical, and therefore more likely to become delinquent than those of thin physique (ectomorphs) or more round, fleshy people (endomorphs) (Sheldon 1949; Glueck and Glueck 1956).
Again, such views have been widely criticized. Even if there were an overall relationship between bodily type and delinquency, this would not necessarily show the determining influence of heredity People of the muscular type may be drawn towards criminal activities because these offer opportunities for the physical display of their athleticism. Moreover, nearly all studies in this field have been restricted to delinquents in reform schools, and it may just be that the tougher, athletic-looking delinquents are more liable to be sent to such schools than fragile-looking, skinny ones. Some individuals might be inclined towards irritability and aggressiveness and this could be reflected in crimes of physical assault on others.