Definition
Sexual intercourse or cohabitation between persons related within the degrees of consanguinity or affinity that prohibit lawful marriage. As a criminal offense, incest targets the act of sexual contact itself, not merely the relationship, and requires that the parties stand in a legally prohibited degree of relation to one another.
Two legal questions structure most incest analysis: (1) which relationships trigger the prohibition, and (2) whether consent is required from both parties for criminal liability to attach. On the second point, the historical sources diverge: most authorities treat incest as requiring mutual assent — distinguishing it from rape — but courts have held that a man may be guilty of incest even where the woman does not consent, meaning the prohibitions can overlap.
Common Language
Modern common usage (Wiktionary): To engage in incestuous sexual activities.
Historical common usage (Webster's 1913): The crime of cohabitation or sexual commerce between persons related within the degrees wherein marriage is prohibited by law. Webster's also recognized "Spiritual incest" in ecclesiastical law: (a) cohabitation between persons united by a spiritual alliance through baptism or confirmation; (b) a vicar or beneficiary holding two benefices dependent on the same collation.
The common understanding of incest tracks the legal definition closely in its core meaning, but diverges in two important respects. First, ordinary usage tends to focus on blood relation (consanguinity), while legal definitions historically extended the prohibition to affinity — relations by marriage — and in ecclesiastical contexts even to spiritual kinship. Second, Webster's ecclesiastical sense of "spiritual incest," long recognized in canon and English ecclesiastical law, has no counterpart in modern criminal statutes and will confuse researchers working in historical ecclesiastical records or early American law that absorbed canonical categories.
Common Confusion
Incest is sometimes conflated with related but distinct offenses. Where the victim does not consent and the prohibited relationship involves a position of familial authority, the conduct may simultaneously satisfy the elements of incest and sexual assault or statutory rape. The offenses are not mutually exclusive, and historical sources occasionally treat them as if they were. Researchers should also distinguish the criminal law of incest from the civil law of prohibited marriages: a marriage between persons in a prohibited degree may be void or voidable under family law statutes without any criminal prosecution, and the degree of relation triggering civil voidness does not always match the degree triggering criminal liability.
Recognized Forms
/SUBTYPES
Incestuous Adultery: Recognized in older authorities as a compounded offense where the parties are both related within prohibited degrees and married to other persons. The elements merge incest with adultery, and some historical jurisdictions treated this as an aggravated form carrying enhanced penalties.
Spiritual Incest: A category from ecclesiastical and canon law, not modern criminal law. Covered (a) sexual relations between persons connected by spiritual kinship through baptism or confirmation, and (b) a clergyman simultaneously holding two benefices under the same patron. Appears in historical ecclesiastical court records and early American jurisdictions that carried forward English ecclesiastical principles.
Why It Matters in Research
The definition of incest in the historical sources is deceptively stable — the language barely changes across dictionaries — but the operative scope shifted considerably depending on whether a jurisdiction defined prohibited degrees by consanguinity alone or also by affinity. Researchers using 19th-century criminal records must check whether the relevant jurisdiction extended the prohibition to in-laws and step-relations, because affinity-based incest was prosecuted in some states and entirely absent in others.
The consent problem is a persistent trap. Bouvier explicitly notes that some courts held incest could be committed by a man even without the woman's consent, creating overlap with rape and assault statutes. Historical indictments and case records may charge both offenses, or may elect one, without making the overlap explicit. A researcher looking for incest prosecutions who searches only incest indictments may miss cases charged under rape or assault statutes where the relationship was the operative aggravating fact.
The ecclesiastical meaning of spiritual incest is a live issue for researchers in colonial American legal history and church court records. English ecclesiastical law was absorbed unevenly into early American jurisdictions, and the category appears in sources that influenced early domestic relations law before being abandoned as criminal codes modernized.
Modern criminal law has trended toward replacing or supplementing incest statutes with sexual abuse and child sexual abuse statutes, which often do not require proof of the prohibited degree of relation. Researchers working in post-1970 criminal records should not assume that incest as a statutory charge is the primary vehicle for prosecuting intrafamilial sexual abuse.
Historical Dictionary Support
The five source dictionaries are in close agreement on the core definition: sexual intercourse or cohabitation between persons related within degrees that prohibit marriage. The uniformity is notable but somewhat misleading, because it papers over the affinity question. Burrill is the only source to explicitly include affinity alongside consanguinity in the structural definition, citing Heineccius for the formulation that incest covers those who "cannot lawfully be united" by reason of either relation. The other dictionaries focus on consanguinity in their lead definitions, with affinity appearing implicitly through the phrase "degrees wherein marriage is prohibited."
Bouvier is the most analytically precise on the consent issue, noting both the general rule that incest requires assent of both parties and the exception recognized in Iowa and California courts allowing conviction of the male without the woman's consent. Black's (2nd ed.) references People v. Stratton and State v. Herges without elaborating on the consent split, so researchers should not treat the case citations as settling the question.
Anderson's Dictionary includes an observation — unusual among the sources — that relationships of age, family authority, or circumstance may overcome a woman's resistance without reaching the threshold of violence required for rape. This is historically significant because it anticipates the modern understanding of coercive incestuous relationships and explains why older courts struggled to categorize cases that fell between consensual incest and forcible rape.
None of the historical dictionaries engage seriously with the jurisdictional variation in prohibited degrees, treating "degrees wherein marriage is prohibited" as if those degrees were uniform. They were not. Researchers should treat the dictionary definitions as identifying the structure of the offense, not the specific relational scope, which requires independent examination of the relevant jurisdiction's marriage prohibition statutes.
Jurisdictional Note
The degrees of relationship triggering incest liability vary by state and have changed over time. Some jurisdictions include half-blood relations, adoptive relations, and step-relations within the prohibition; others do not. A handful of modern states have substantially narrowed or restructured their incest statutes in response to constitutional challenges and the expansion of overlapping sexual abuse offenses. Researchers should never assume that the prohibited degrees identified in one jurisdiction's sources apply to another.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia — Sexual Offenses (for criminal law framework and modern statutory treatment); Marriage Prohibitions and Void Marriages (for civil law overlap and prohibited degrees); Consanguinity and Affinity (for relational classifications underlying both criminal and civil prohibitions).