Definition
Prohibited degrees are the degrees of kinship — by blood (consanguinity), marriage (affinity), or in some jurisdictions adoption — within which two persons are legally barred from marrying one another. The prohibition operates on the premise that certain relationships are too close for marriage to be permitted, whether on grounds of biology, social order, or moral tradition.
The "degrees" refer to the steps of relationship counted between two persons on a genealogical table. The closer the relationship, the lower the degree number; the law prohibits marriage within whatever range the relevant statute or canon law draws. First-degree relations (parent-child, sibling-sibling) are universally prohibited. Beyond that, the cutoff varies sharply by jurisdiction and era.
The term encompasses two overlapping categories:
1. Consanguinity prohibitions — relationships by blood, including lineal (ancestor to descendant) and collateral (cousins, aunts, uncles, nephews, nieces).
2. Affinity prohibitions — relationships created by marriage rather than blood, such as a stepparent or a deceased spouse's sibling. These have contracted significantly in modern law and are now largely abolished or narrowed in most common-law jurisdictions.
Common Confusion
Prohibited degrees is sometimes used interchangeably with incest, but the two are not identical. Incest is the criminal offense; prohibited degrees describes the civil marriage bar. A relationship may fall within the prohibited degrees without resulting in criminal prosecution, and historically the definitions did not always overlap. Canon law prohibited degrees that secular criminal law did not reach, and vice versa. Researchers working across ecclesiastical and civil sources must be attentive to which regime's prohibited degrees are at issue.
The related concept of consanguinity is also distinguished from affinity: consanguinity is blood relationship; affinity is relationship through marriage. Many historical prohibited-degrees tables listed both, but modern statutes have largely stripped affinity prohibitions from the list, making the historical and modern scopes of the term quite different.
Why It Matters in Research
This term is a navigational minefield in historical sources for several reasons.
First, the scope has contracted dramatically. Pre-twentieth-century sources — and especially ecclesiastical records and treatises — operate under a much broader set of prohibited degrees than modern statutes recognize. The Church of England's Table of Kindred and Affinity (1560, revised 1946) listed twenty-five prohibited relationships, many based on affinity that have since been abolished in England and Wales. A researcher reading a nineteenth-century case or treatise will encounter prohibited degrees that no longer carry legal weight and must not assume modern equivalence.
Second, jurisdiction determines everything. American states set their own prohibited degrees by statute, and the variation is significant — some states prohibit first-cousin marriage, others permit it. England progressively narrowed affinity prohibitions through the Deceased Wife's Sister's Marriage Act 1907 and subsequent legislation. Comparative research requires identifying the specific jurisdiction and date before drawing conclusions.
Third, the term appears in both civil and ecclesiastical court records. Before civil marriage registration became standard, church courts adjudicated prohibited degrees questions. Researchers in colonial American or English ecclesiastical records will find the canon law framework (based on Levitical degrees), not the later statutory one.
Fourth, the consequences of a marriage within the prohibited degrees have shifted. Historically, such marriages were voidable (capable of annulment) rather than void ab initio in many jurisdictions; in others they were treated as absolutely void. This distinction affects property rights, legitimacy of children, and inheritance — all of which appear in probate and equity records that researchers may consult.
Historical Dictionary Support
Black's Law Dictionary (2nd Ed.) defines prohibited degrees as "those degrees of relationship by consanguinity which are so close that marriage between persons related to each other in any of such degrees is forbidden by law." The entry is brief and, notably, frames the prohibition exclusively in terms of consanguinity — omitting affinity. This reflects neither universal historical practice nor Black's oversight, but rather a simplification that researchers should not take as comprehensive. Historical and ecclesiastical sources treated affinity prohibitions with equal seriousness, and many nineteenth-century American statutes expressly covered both.
Black's cites State v. Guiton, 51 La. Ann. 155, 24 So. 784 — a Louisiana case — which is consistent with the civil-law influence on Louisiana's treatment of marriage prohibitions, distinct from the common-law states. Researchers relying on Black's alone for this term will miss the affinity dimension and the canon law dimension entirely.
No Bouvier's or Wharton's entry is available in the source set for this term, but both treatises historically gave the subject considerably more space, mapping out the degrees by table and addressing the ecclesiastical framework. Their absence here is a gap.
Jurisdictional Note
American states define prohibited degrees by statute, and meaningful variation exists: first-cousin marriage is permitted in approximately half of U.S. states and prohibited in the others. Affinity prohibitions have been substantially abolished in most states but survive in modified form in a few. In England and Wales, the Marriage (Prohibited Degrees of Relationship) Act 1986 significantly restructured the rules, permitting certain formerly prohibited affinity relationships under specified conditions.