No, it is illegal to marry your first cousin in South Dakota under state law, which declares such unions null and void from the beginning. This applies to both full and half-blood first cousins, with no exceptions based on age or fertility as of March 2026.
Legal Prohibition
South Dakota Codified Laws § 25-1-6 explicitly voids marriages between cousins of the half or whole blood, alongside closer relatives like siblings, parents/children, uncles/nieces, and aunts/nephews. These relationships include those by adoption, ensuring broad coverage. Second cousins and more distant kin (e.g., first cousins once removed) face no such ban, allowing those unions legally.
Historical Context
The ban dates to the 1860s territorial laws and remains unchanged, placing South Dakota among 25 states restricting first-cousin marriage—eight treating it as criminal. No 2026 legislative reforms noted; marriage licenses require affirming no impediments, with county registers rejecting prohibited applicants.
Penalties and Enforcement
Attempted marriages are automatically void, not merely voidable, preventing legal recognition even post-ceremony. No direct criminal penalty for marrying (unlike sexual relations in some states), but fraud on license apps risks misdemeanor charges or fines up to $500. Children from such unions hold legitimacy status but may face inheritance scrutiny.
| Cousin Type | Marriage Allowed? | Notes |
|---|---|---|
| First (full/half-blood) | No | Void ab initio; adoption included |
| First once removed | Yes | Child of first cousin |
| Second | Yes | Parents are first cousins |
| Distant (third+) | Yes | No restrictions |
Out-of-State Marriages
A South Dakota-issued marriage to a first cousin elsewhere (where legal) may gain partial recognition for residency purposes, but courts void it for divorce, inheritance, or benefits under comity principles favoring local incest laws. Residency tests apply; move out first.​
Genetic and Social Factors
Bans stem from 19th-century eugenics concerns over birth defects (3-5% higher risk in first-cousin offspring vs. 2-3% general). Modern genetics views risks low for adults without inbreeding history. Globally, cousin marriage common (10% worldwide); U.S. stigma persists in restrictive states.
Practical Steps
Verify relation via genealogy (e.g., Ancestry DNA). For allowed cousins, apply at any county register of deeds (no blood test, 3-day wait optional). Age 18+ required; 16-17 needs notarized parental consent. Consult attorney for edge cases like step-relations.
SOURCES:
- https://sdlegislature.gov/Statutes/25-1
- https://en.wikipedia.org/wiki/Cousin_marriage_law_in_the_United_States
