Inbreeding Statistics by Race: What the Data Show

Nobody measures inbreeding by race. Researchers count marriages between relatives by country, region and community, so the figures people search for under that label don’t exist. What the numbers describe is a custom, one that some cultures favor and others have almost abandoned, not a trait of any race.1Nature & Anthropology. Consanguineous Marriage in Global Perspective: Anthropological Roots, Genetic Risks, Contemporary Relevance, and the Way Forward Worldwide, about 1 in 10 people are in, or were born to, a marriage between second cousins or closer.2PubMed Central. Consanguinity, human evolution, and complex diseases A population can also share a lot of DNA without anyone marrying a cousin, simply because its ancestors were once few in number, so genetic sameness and cousin marriage are different measures.

Cousin Marriage Rates Around the World

First cousins are the most common pairing, making up nearly a quarter of all marriages in many Arab countries.8PubMed Central. The Determinants of Consanguineous Marriages among the Arab Population: A Systematic Review In one survey in northern Morocco, 45.4% of couples were related, and 91% of those were first cousins.9PAMJ Clinical Medicine. Analysis of some determining factors of consanguinity in North Morocco: a cross-sectional epidemiological study More than half of those cousin marriages were to a daughter of the father’s brother.10PAMJ Clinical Medicine. Analysis of some determining factors of consanguinity in North Morocco: a cross-sectional epidemiological study The highest rates overall are in north and sub-Saharan Africa, the Middle East, and west, central and south Asia.11PubMed Central. Consanguinity, human evolution, and complex diseases

A rate is the share of marriages, not the share of people with health problems. Rates for one province or survey can differ sharply from the national figure, and small local surveys show what is possible in a community rather than in a whole country.

The list has no race column because each figure covers a region or country containing many ethnic and religious groups. The low-rate regions include countries where the practice was once common, like Spain.

Most figures come from household surveys or clinic and hospital records, where couples are asked whether they are related and how. They depend on what people know and are willing to say. Surveys also define “related” differently: some count only first cousins, others second cousins or closer, so two countries’ figures are not always directly comparable. Many countries have no national survey at all, so some regional estimates rest on a few studies and carry real uncertainty.

Why Race Is the Wrong Way to Sort the Data

In Sri Lanka, ethnic Tamils marry relatives at about 22.4%, compared with 3.8% among Sinhalese and 3.2% among Moors.12Journal of Biosocial Science. Marriage patterns in Sri Lanka and the prevalence of parental consanguinity in patients with β-thalassaemia: a cross-sectional descriptive analysis Arranged marriages there were about twice as likely to be between relatives as love matches, 11.7% versus 5.6%.13Journal of Biosocial Science. Marriage patterns in Sri Lanka and the prevalence of parental consanguinity in patients with β-thalassaemia: a cross-sectional descriptive analysis

It is a common mistake to equate “Muslim,” “Arab” or “Asian” with high rates. Sri Lanka’s Muslim Moors have one of the lowest rates on the island, and South Asia’s rates swing widely between castes, regions and religions. The deciding factor is usually family custom, not ancestry or skin color. A better sorting question asks which community, with which religion, caste and family customs, in which region, and whether the families live in cities or villages.

The arranged-marriage gap makes sense when you look at how matches are made. Families arranging a marriage often look first within the wider family, while couples choosing their own partners usually meet outside the family network. So as love matches spread, cousin marriage tends to fall.

Where people have long married inside their clan, tribe, caste or biraderi (extended kin group), even couples who see themselves as unrelated can share a lot of DNA.14PubMed Central. Consanguinity, human evolution, and complex diseases Official cousin-marriage rates can therefore understate how related couples really are. In communities where marriage is expected within a caste or clan, the pool of acceptable partners may be only a few hundred or thousand people, so spouses are often distant relatives even when not cousins. Some families go further and prefer a cousin, adding close-relative marriage on top of group-level relatedness.

The custom also travels with families, so emigrant communities in North America, Europe and Australia practice it too.15PubMed Central. Consanguineous marriages A national rate for, say, Britain hides much higher rates in particular immigrant communities and much lower rates elsewhere.

Shared DNA Without Cousin Marriage

Sometimes a child inherits the same long stretch of DNA from both parents.16Nature Reviews Genetics. Runs of homozygosity: windows into population history and trait architecture Cousin marriage creates these, but they turn up in everyone, including people whose parents are not related.17Nature Reviews Genetics. Runs of homozygosity: windows into population history and trait architecture Their number and length record a person’s family and population history.18Nature Reviews Genetics. Runs of homozygosity: windows into population history and trait architecture Long matching stretches point to relatives marrying recently, while many short ones point to a small population far in the past.

Length matters because DNA is shuffled and chopped every generation, so a shared stretch inherited from an ancestor gets shorter as the generations pass. A long stretch therefore points to a shared ancestor only a few generations back, such as a grandparent common to both parents. Short stretches come from ancestors much farther back, and everyone has some, so they say nothing about a couple’s recent marriage choices.

The Himba of northern Namibia carry raised levels of matching DNA, and their customs favor cousin marriage.19PubMed Central. Increased homozygosity due to endogamy results in fitness consequences in a human population Yet genetic testing found no actual first-cousin couples among them; their sameness traces to a population crash about 12 to 18 generations ago.20PubMed Central. Increased homozygosity due to endogamy results in fitness consequences in a human population Pacific Island populations have many short matching stretches but few long ones, showing a small population long ago but little recent inbreeding.21PubMed Central. Genomic Runs of Homozygosity Record Population History and Consanguinity

A population crash explains these cases. When a group shrinks to a few people, everyone later descends from that small pool, so their descendants share DNA even if each couple is unrelated in living memory. The effect fades only slowly as the group grows and mixes.

A fair reading of a headline about a group’s DNA asks whether the study measured long or short stretches. It also asks whether researchers checked family records or interviewed couples, as in the Himba study, rather than inferring cousin marriage from DNA alone.

What Inbreeding Does to Health

Related parents are more likely to carry the same harmful gene from a shared ancestor, and a child who gets two copies can be ill.22Egyptian Journal of Medical Human Genetics. Consanguinity and its relevance to clinical genetics In one clinic’s patients, 78.8% of those with recessive disorders had related parents, versus 29.1% for chromosome disorders like Down syndrome.23Egyptian Journal of Medical Human Genetics. Consanguinity and its relevance to clinical genetics These conditions can cause miscarriage, infant death or childhood illness.24Egyptian Journal of Medical Human Genetics. Consanguinity and its relevance to clinical genetics

Recessive means a gene needs two copies to cause illness. Everyone carries a few harmful recessive variants, and with one copy and one working copy a person is usually healthy and unaware. A child is affected only when both parents pass on the same variant, which is unlikely for unrelated parents because they rarely carry the same rare one. Cousins descend from the same grandparents, so a variant in one grandparent can reach both parents, raising the chance that both pass it on.

Most children of first cousins are born healthy. The added risk is real but concentrated in recessive conditions, and it is higher when a family has married relatives for many generations. Knowing whether a specific disorder runs in the family changes the picture most.

Everyday traits are affected slightly. More shared DNA is linked to slightly shorter height, driven by many rare genes each with a tiny effect.25PubMed Central. Evidence of Inbreeding Depression on Human Height In a large UK study, inbreeding was linked to weaker grip, a higher waist-to-hip ratio and poorer vision and hearing.26PubMed Central. Detection and quantification of inbreeding depression for complex traits from SNP data Long matching stretches carry more harmful mutations, because they haven’t had time to be weeded out over generations.27PubMed Central. Inbreeding and deleterious variation in South Asian populations

Risk also adds up over generations. An inbreeding coefficient is a score for how much of a person’s DNA is expected to match from both parents; for a child of first cousins it is 0.0625. Spain’s Habsburg kings rose from 0.025 to 0.254 for Charles II over generations.28PubMed Central. The Role of Inbreeding in the Extinction of a European Royal Dynasty That matched what is expected from a brother-sister union, though his parents were uncle and niece.29PubMed Central. The Role of Inbreeding in the Extinction of a European Royal Dynasty Repeated marriages among more distant relatives contributed about as much as the close marriages.30PubMed Central. The Role of Inbreeding in the Extinction of a European Royal Dynasty

Why Some Communities Prefer Cousin Marriage

Limited access to education and money strongly shapes partner choice.31Genealogy. Social and Demographic Determinants of Consanguineous Marriage: Insights from a Literature Review Rural life is linked to higher rates, mainly because it goes with less schooling, earlier marriage and lower income.32European Journal of General Medicine. Prevalence and Risk Factors of Consanguineous Marriage In some places the practice is a cultural norm, while in Western nations it has nearly vanished.33Nature & Anthropology. Consanguineous Marriage in Global Perspective: Anthropological Roots, Genetic Risks, Contemporary Relevance, and the Way Forward

Families see real benefits. A bride marries into a family she already knows, dowry and land stay within the family, and negotiations are simpler because trust is built in. These reasons explain why bans alone rarely change behavior quickly.

Where land is the main source of income and is divided among heirs, marrying within the family keeps a holding from being split across households. A marriage between cousins can even reunite property that earlier generations divided.

City life changes the choice. Jobs and schooling widen the circle of possible partners, young people increasingly meet spouses on their own, and families depend less on land that must be kept together.

Is Cousin Marriage Declining

Overall popularity has declined, especially in wealthy countries.34Annual Review of Anthropology. Consanguineous Marriage and Human Evolution Spain had some of Europe’s highest rates into the 1950s and 1960s, then they fell fast and close-relative marriage became rare.35PubMed. The Evolution of Consanguineous Marriages in the Archdiocese of Granada, Spain (1900-1979) In northwest Pakistan, rates fell from marriages begun before 1980 to those after 2010, as literacy rose, people married later and exchange marriages between families fell.36PubMed. Consanguinity in northwest Pakistan: evidence of temporal decline India, with about 18% of the world’s people, shows mixed trends across its many groups.37Journal of Biosocial Science. Has the long-predicted decline in consanguineous marriage in India occurred? Earlier claims of falling genetic sameness came from small, mostly non-diverse samples.38PubMed Central. Declining autozygosity over time: An exploration in over 1 million individuals from three diverse cohorts

Education, age at marriage and exchange marriages move together. As women stay in school longer, they marry later and have more say in who they marry, which reduces the pull of a match arranged inside the family. Exchange marriages, where two families swap brides, usually bring in relatives or neighbors, so their decline removes a common route to cousin marriage.

A slow decline takes a generation to show in children’s health, and genetic sameness from past generations lingers even after the custom fades. Younger cohorts may be far below the average for all adults, and rates can fall in cities while staying high in rural districts, which keeps the national figure high.

Screening and Laws on Cousin Marriage

Premarital genetic screening programs have been highly effective at preventing genetic disorders in the Gulf region and Middle East.39PubMed Central. Comprehension of premarital screening and genetic disorders among the population of Riyadh In Sudan, 85.2% of people surveyed thought screening would reduce genetic disease, and 82.3% of unmarried respondents were willing to be tested.40PubMed Central. Consanguinity and willingness to perform premarital genetic screening in Sudan

Carrier testing is a blood or saliva test that checks both partners for the same recessive genes. If both carry the same one, each child faces a chance of the condition, and genetic counselors explain options such as testing during pregnancy.

Testing is most useful before pregnancy, so couples have time to understand results and consider their options. It is especially worth considering if the couple are related, come from a community with a high rate of a particular inherited disorder, or have a relative with a known recessive condition. A negative result lowers the risk for the conditions tested but cannot rule out every genetic disorder.

Rules on cousin marriage vary widely between countries and have changed over time.41Research Handbook on Marriage, Cohabitation and the Law. Comparative consanguinity law: A global study of cousin marriage regulations Norway banned cousin marriage entirely from September 2023.42PubMed Central. Consanguineous Marriage: Law and Public Health Screening lets couples marry while knowing their specific risk, whereas bans target the practice but may push it out of sight. Many health experts favor counseling because it respects family choice.