The average coefficient of inbreeding in metropolitan France remains among the lowest in the world. This value, often cited without context, masks regional and methodological disparities that recent genomic analyses allow us to better understand. Here, we provide a technical breakdown of the available data and its structural limitations.
Coefficient F and runs of homozygosity: what genomics corrects
Classic estimates of the coefficient of inbreeding (F) in France rely on ecclesiastical dispensations and civil registry records. These sources, utilized by demographers since the works published in Population between 1926 and 1958, produce a very low average F. Genomics provides a notable correction.
The runs of homozygosity (ROH), chromosomal segments where both parental copies are identical, allow for the quantification of actual inbreeding without relying on declared genealogy. An individual from first cousins presents long and numerous ROH, while ancient inbreeding (several generations back) produces short but dispersed ROH across the genome.
As explained by the inbreeding rate in France 2026 according to Esprit Maman, the distinction between these two genomic profiles changes the interpretation of raw figures. An F calculated through genealogy underestimates ancient inbreeding and ignores undeclared unions.
We observe that private genomic databases, fueled by commercial tests conducted abroad (their sale remaining prohibited in France), now provide sufficient samples to map ROH at the regional level. This quasi-official genetic registry contradicts the absence of centralized demographic tracking of consanguineous unions.
Rate of consanguineous marriages in France: a demographic indicator not to be confused with F
The rate of consanguineous marriages accounts for the proportion of unions between related individuals within a population. In France, this rate is estimated at a few percent according to recent sources. This figure aggregates very different realities.
Insular regions and enclosed valleys maintain higher rates than the national average. Corsica regularly appears in analyses as an area where historical endogamy has left a measurable genomic footprint. The Pyrenean valleys and certain mountainous areas present comparable profiles, linked to centuries of geographical isolation.
Furthermore, some communities from immigration maintain practices of cousin unions, common in their countries of origin. These marriages fall under close inbreeding (high F by union), unlike insular endogamy which produces diffuse inbreeding over several generations.
Why the two phenomena are not treated the same way
Ancient geographical endogamy moderately increases the risk of autosomal recessive diseases. Close inbreeding (first cousins) doubles this risk as documented in the international medical literature, as confirmed by studies conducted on the Moroccan population with a frequency of consanguineous marriages exceeding 40% in certain studied municipalities.
We recommend never to aggregate these two situations into a single figure: a low average national rate may mask pockets of real genetic risk.
French legal framework and prohibitions on marriage between relatives
The Civil Code prohibits marriage between direct ascendants and descendants, between siblings, and between certain in-laws. These prohibitions extend to blended families, a point clarified by recent developments in family law jurisprudence.
- Marriage prohibited between parent and child, grandparent and grandchild, with no exceptions or dispensations possible
- Marriage prohibited between brother and sister, half-brother and half-sister
- Marriage between uncle/aunt and nephew/niece, or between first cousins, subject to a dispensation regime by the President of the Republic for the former, authorized without formalities for the latter
- Extension of prohibitions to in-laws in blended families (step-parent/step-child) as long as the marriage creating the alliance subsists
The penal dimension comes into play when incestuous relationships involve minors, thus falling under aggravated sexual offenses. Incest constitutes a systematic aggravating circumstance in the penal code, regardless of the question of marriage.
Genetic genealogy and criminal investigations: the law of July 2026
The law on criminal justice adopted on July 9, 2026, allows for the use of genetic genealogy in criminal investigations. This provision, reserved for serious unsolved crimes for more than five years, requires the authorization of the investigating judge or the judge of freedoms and detention.
This legislative advancement paves the way for a large-scale indirect exploitation of kinship links. Investigators can now request the consultation of genomic databases to identify strongly related family clusters, marking a paradigm shift in detecting inbreeding links within targeted populations.
The paradox is clear: France has no official registry of consanguineous unions, but the judiciary can now exploit private genetic databases established abroad. This asymmetry between the absence of health monitoring and the new judicial capacity raises questions that neither the legislator nor geneticists have yet resolved.
International comparison of the rate of inbreeding
France is situated in the lower range globally. Some countries in the Middle East, North Africa, and South Asia exhibit rates of consanguineous marriages that far exceed European averages. In Pakistan, the proportion of consanguineous unions exceeds half of marriages in certain districts.
- Western Europe: rates generally below a few percent, with localized pockets of endogamy
- Middle East and North Africa: rates often exceeding one in four marriages, even one in two in certain regions
- South Asia: high variability depending on communities, with very high rates in rural areas of Pakistan and southern India
The global average is around one in ten marriages, placing France significantly below. This position does not exempt from targeted vigilance on at-risk populations identified by genomic data.
The absence of a dedicated national registry remains the main blind spot of the French system. Data exist, scattered among private databases, one-off studies, and hospital registries of rare diseases. Connecting them within a coherent epidemiological framework would allow for a shift from a vague estimate to operational monitoring, without this perspective currently being part of an identified public health program.