Definition
Consanguinity is the legal relationship between persons descended from the same common ancestor — blood relationship, as distinguished from affinity, which arises from marriage. It is the foundational concept for determining whether persons share a family connection through biological descent rather than legal or marital ties.
Consanguinity takes two recognized forms:
1. Lineal consanguinity: The relationship between persons in a direct line of descent — one is the ancestor of the other. This runs both upward (child to parent to grandparent) and downward (parent to child to grandchild). The line is continuous and direct.
2. Collateral consanguinity: The relationship between persons who share a common ancestor but are not in a direct line of descent from one another. Siblings, cousins, aunts, and uncles are collateral relatives — connected through a shared ancestor, not through direct descent from one another.
Consanguinity is used across multiple legal contexts: inheritance and intestate succession (determining who qualifies as an heir), marriage law (establishing prohibited degrees within which marriage is void or voidable), guardianship and custody (preferring blood relatives), and evidence (governing witness competency and privilege rules in some jurisdictions).
Common Language
Modern common usage (Wiktionary): A consanguineous or family relationship through parentage or descent; a blood relationship; also, inbreeding.
Historical common usage (Webster's 1913): The relation of persons by blood, in distinction from affinity or relation by marriage; blood relationship, as lineal consanguinity or collateral consanguinity.
The common-language definitions closely track the legal definition, which is unusual. The meaningful gap lies not in the core concept but in application: in ordinary usage, consanguinity signals biological relatedness; in law, it triggers specific legal consequences — prohibited marriages, inheritance rights, priority in guardianship — and is measured by precise degree calculations that have no common-language equivalent.
Core Elements
Consanguinity as a legal concept requires establishing:
1. Common ancestor: Two persons must share a biological ancestor from whom both are descended, either directly or through a branching line.
2. Line of descent: Whether the relationship is lineal (one descended from the other) or collateral (both descended from a shared third party) determines which legal rules apply.
3. Degree of relationship: The closeness of the blood relationship is measured in degrees, calculated either by the civil law method (counting the number of generational steps from common ancestor to each relative) or the canon law method (counting only the longer of the two lines). The method of calculation matters because prohibited-marriage statutes and inheritance statutes historically specified degrees.
4. Legitimacy (historically): Some historical sources conditioned legal consanguinity on legitimate birth. Modern law has substantially eliminated this distinction.
Recognized Forms
/SUBTYPES
LINEAL CONSANGUINITY: Direct line of descent. One party is the ascendant or descendant of the other. The relationship is vertical: grandparent–parent–child–grandchild. No degree of lineal consanguinity is permissible for marriage in any American jurisdiction.
COLLATERAL CONSANGUINITY: Shared ancestor but no direct descent between the parties. The relationship branches from a common point. Siblings (first degree collateral), first cousins (second degree collateral), and more remote relations fall here. Legal consequences — particularly marriage prohibitions — depend on the degree of collateral relationship, which varies significantly by jurisdiction.
Why It Matters in Research
The most important navigational point: historical legal sources use two different systems for calculating degrees of consanguinity, and they produce different numbers for the same relationship. The civil law method counts each generational step (parent and child are one degree; siblings are two degrees because you count up to the common ancestor and back down). The canon law method counts only the longer line to the common ancestor. A first-cousin marriage is "fourth degree" under civil law and "second degree" under canon law. When reading historical statutes or cases, identifying which computational method applies is essential — a prohibition on marriage within the "fourth degree" means something different depending on the system in use.
Researchers working in succession law will find consanguinity operating differently than in marriage law. Intestate succession statutes use degree calculations to rank heirs by proximity, but the specific rules for how degrees are counted and how collateral lines are weighted vary by state and era. Historical equity courts sometimes applied different consanguinity rules than courts of law in the same jurisdiction.
The distinction between consanguinity and affinity is a persistent research trap. Affinity — relationship by marriage — was historically treated as a separate but parallel bar to marriage and sometimes to inheritance. Many historical sources discuss both in close proximity, and older statutes prohibiting marriage "within certain degrees" encompassed both consanguineous and affinal relationships. Do not assume a prohibition on "relatives" is limited to blood kin.
For researchers in family law: modern prohibited-marriage statutes have largely replaced the degree framework with explicit lists of forbidden relationships. Historical sources structured around degree calculations may not map cleanly onto modern statutory text, even when the underlying policy is identical.
Historical Dictionary Support
The historical dictionaries converge cleanly on the core definition. Burrill's is the most precise, defining consanguinity as "the connection or relation of persons descended from the same stock or common ancestor" and sharply distinguishing it from affinity as "relation by blood, as affinity is relation by marriage" — a formulation traceable directly to Blackstone. Burrill also supplies the Latin maxim *vinculum personarum ab eodem stipite descendentium* (the bond of persons descending from the same stock), useful for reading Latin-embedded common law sources.
Black's (both editions) mirrors Blackstone's Commentaries closely, providing the lineal/collateral framework with illustrative examples running through ascending and descending lines. The entries are descriptive rather than analytical — they define the taxonomy but do not address degree calculation methods or jurisdictional variations in prohibited-degree rules.
Bouvier's contextualizes consanguinity primarily through succession and property law, reflecting its treatise orientation. Anderson's contribution in the available extracts is minimal for this term specifically.
What the historical dictionaries largely omit: the mechanics of degree calculation, the civil law versus canon law divergence, and the treatment of illegitimate children — all of which were practically significant in historical litigation. Researchers relying solely on dictionary definitions will need to supplement with Blackstone's Commentaries (Book II, Chapter 1) and period succession treatises for computational method.
Jurisdictional Note
Marriage prohibitions based on consanguinity vary by state. All American jurisdictions prohibit marriage between lineal relatives and between siblings. First-cousin marriage is permitted in roughly half of U.S. states and prohibited in the rest. Some states distinguish between first cousins once removed or double first cousins. Historical prohibited-degree tables based on canon law degrees do not translate directly into modern statutory language without checking the applicable jurisdiction's current statute.
Encyclopedia Cross-Reference
Marriage — Capacity to Marry (Age, Consanguinity, Affinity) (The Law Mind Family Law Encyclopedia)