Fosterage was the medieval Irish custom by which the parents of a child would send him or her to be raised and educated by another family. Two main categories of fosterage are discernible: fosterage for affection or fosterage for a fee. In cases of fosterage for a fee (higher for girls because they were considered less beneficial to the foster parents), costs were determined by the social standing of the child’s father.
The purpose of fosterage was to cultivate closer ties between the two families. It could be used to strengthen marriage ties through fosterage with the child’s maternal line or to form or reinforce ties through fosterage with allies or vassals. Its effectiveness in this capacity was due to the strong bond that often developed between the child and his foster parents and siblings. This bond was reflected in the Law Tracts that show that the child had obligations to support his foster parents in their old age, and should a fostered child be murdered, his foster family had a right to part of his honor price. Even the right and responsibility to avenge the murder of a foster son was extended to the foster family.
The age at which fosterage began varied widely; it could begin as early as infancy or as late as age ten. The age at which fosterage concluded seems to have been more formalized. Although there are indications that fosterage for both sexes could be considered complete at age fourteen or seventeen, it has been suggested that the most common custom was that girls remained in fosterage until age fourteen when they could marry and boys remained in fosterage until age seventeen, the age of maturity. During the period of fosterage, the foster parents were responsible for raising, educating, and maintaining the child in a manner appropriate to the social standing of the child’s father; for example, a king’s son was to be taught martial skills but a boy of lower rank was to learn the skills necessary for farming and animal husbandry. Even the child’s diet reflected his rank—gruel and buttermilk being the daily staple of commoner children, while nobles enjoyed luxuries such as wheaten porridge and honey.
Following the Anglo-Norman Invasion, the Anglo-Normans adopted Irish customs such as fosterage and gossiprid to establish alliances with the Irish. By the fourteenth century, the adoption of these Irish customs had become a point of concern for the royal government because of the divided loyalties they engendered and because they were seen as one of the causes of Gaelicization. Laws, such as the Statutes of Kilkenny (1366), were passed outlawing the practice, but these laws appear to have had only limited effect, and they were undermined by the royal government’s willingness to grant exemptions.
Fosterage continued to be practiced throughout the Middle Ages, but by the end of the sixteenth century the term referred to an even wider range of relationships, including purely financial arrangements wherein the "foster family" did not actually take custody of the child but rather paid a yearly sum to the child and fulfilled the traditional financial obligations of fosterage.