Throughout Hieradoumia, we find many hundreds of instances of people commemorating and being commemorated by their foster-children (threptoi), foster-parents (threpsantes), and foster-siblings (syntrophoi). The ‘rearing’ of non-natal children was so ubiquitous in Roman Hieradoumia that fosterage appears to have been a standard familial strategy for circulating children temporarily or permanently between households, rather than necessarily a response to orphanhood or extreme familial dysfunction. Foster-children could be of either higher or lower social status than their foster-parents; in a few cases, there is reason to think that children were reared by close relatives (particularly the natal parents’ siblings). It is argued that one of the social functions of fosterage was to cement ties or alliances between family groups; the word synteknos may be a technical term for the relationship between natal parent and foster-parent. Sentimental relations between foster-kin were often very close, and we often find foster-kin assimilated to natal kin.

Informal institutions are the voluntary social arrangements established between households that are used to solve problems that they cannot resolve on their own. Examples from seven ancient and premodern societies are used to illustrate the operation of informal institutions to mobilize labor, establish and maintain interhousehold social networks, obtain spouses, construct intergroup trade networks, and supply emergency support to avoid household failure.