The Formal Demography of Affinal Kin: Marital Dynamics and Union-Linked Kin Networks

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Kinship networks are critical sources of rights and obligations that shape individuals’ relationships and access to resources over the life course. In recent years, formal demographic models of kinship have garnered considerable interest, as they provide a powerful framework for describing how these networks emerge from fertility and mortality processes and quantifying aspects of contemporary kinship of relevance in several applications such as caregiving, social support, social capital, and bereavement. Existing formal demographic models of kinship primarily represent biological kin and do not explicitly model the union links that connect focal individuals to spouses and their respective kin networks. This omission is consequential because unions connect individuals to broader networks of familial relationships, rights, and obligations that extend beyond biological kinship, a key insight of contemporary social demography. We introduce a new formal demography of affinal kin, which offers important first steps for going beyond biological kinship networks. Building on the multistate kinship matrix modeling framework, we use marital status as a stage and introduce partner-age pairing and spouse-link operators to connect currently married individuals to spouses and union-linked kin relationships. The model we introduce distinguishes current spouses, spouse-side biological kin, and spouses of biological kin, allowing biological and union-linked kin to be projected jointly over the life course. We illustrate this new formal model using 2023 U.S. marriage, fertility, and mortality schedules under a first-marriage, no-remarriage baseline to demonstrate the promise of this approach. Results show that union-linked kin constitute a substantial share of available kin and follow distinct age profiles from biological kin, adding considerable new understandings about kinship structure. This new framework provides a foundation for future extensions to former affinal ties, remarriages, step-kin, and more complex union histories.