In the twelfth and early thirteenth centuries, the Church succeeded in gaining jurisdiction over marriage and transforming it from a largely private arrangement between families to the expression of a Christian ideal, subject to a codified system of regulation shaped by theological concerns. The conception of marriage as an earthly expression of the bond between Christ and the faithful, of the family as a microcosm of wider social polities reflective of the heavenly regime, and of both as a means of maintaining a peaceful and well-ordered society, combined to produce in authorities both secular and ecclesiastical a belief that marriage contracted according to the laws of the Church was the only acceptable context for the founding of a family.

Previously, kingdoms and the population groups within them had their own definitions and rules of what constituted marriage, and, independently of this, which children could be heirs. In many early medieval societies the offspring of more than one woman might be heirs to their father. It is not unreasonable to suppose that in at least some cases ‘legitimacy’ worked backwards – those children selected by their fathers as heirs, or for other honours such as prestigious marriages, may have been deemed ‘legitimate’ by the fact of being selected, not by the pre-existing nature of the relationship between their parents.

Only with a clear definition of marriage can offspring be identified as legitimate or illegitimate. Still, the meanings attached to these terms, the consequences of being legitimate or not, depend on how society decides to acknowledge the distinction, or on whether it does so at all. Scotland, along with many other kingdoms, enshrined in secular legislation the legal disability of bastardy: the principle that illegitimate people may not be heirs, nor have heirs other than their own, lawfully procreated, offspring. Wealth, power, prestige – or, in many cases, subsistence and the hope of future improvement to material circumstances – rested on the precise nature of an individual's relationship to land and property. To be barred from inheriting land, and the income derived from it, was to suffer serious disadvantage. The prohibition on inheritance was not imposed on Scottish society by the Church, which, though vigorous in promoting monogamous marriage, was not interested in penalising illegitimates among the laity.