Home / Blog / Cousin marriage side effects: what the science says

Cousin marriage:
what the science actually says.

Cousin marriage is legal in many countries, permitted in Islam and common across large parts of the world. It's also surrounded by myths in both directions. Here are the real numbers on health risks, why they rise over generations, and what couples can do.

By the DontMarryYourBro team · Updated October 2026 · 9 min read · How we fact-check

The short answer

  • For first cousins, the chance of a serious birth defect roughly doubles: from about 3% to about 6%. Most children of first cousins are healthy.
  • In Bradford, UK, marriage between relatives accounted for about 31% of birth anomalies among babies of Pakistani heritage (The Lancet, 2013).
  • Risk climbs when cousin marriage repeats across generations. Even "distant" relatives in those families can carry first-cousin-level risk.
  • A carrier screen tells a specific couple whether they actually share a condition, turning a general statistic into a personal answer.

How common is cousin marriage?

  • Worldwide: couples related as second cousins or closer, and their children, make up about 10% of the world's population.
  • Pakistan: in the 2017–18 national survey, about 63% of married women had married a blood relative, and about half had married a first cousin.
  • India: about 11% of marriages are consanguineous (NFHS-5), rising to roughly 27–28% in Tamil Nadu and Karnataka.
  • Saudi Arabia: about 56% of mothers in a 2004–05 national study were married to a relative.
  • UK: in Bradford, 37% of babies of Pakistani heritage born in 2007–11 had first-cousin parents. By 2016–19, first-cousin marriage among Pakistani-heritage mothers there had fallen from about 39% to 27%.

Why cousins have a higher risk

Everyone carries a few silent gene variants that are harmless on their own. A child is only affected by a recessive condition when they inherit the same variant from both parents. Cousins share grandparents, so they're more likely to carry the same variant from the same ancestor.

Cousin marriage doesn't create new genetic changes. It raises the chance that two carriers of the same condition have children together.

The real numbers

  • Born in Bradford (The Lancet, 2013): of 11,396 babies, 3% had a congenital anomaly. Marriage between relatives roughly doubled the risk, and that held after accounting for deprivation. The study's lead author summed it up as an increase "from three percent to six percent."
  • The same study estimated that about 31% of anomalies in babies of Pakistani heritage were attributable to consanguinity. It also found a similar rise in risk among White British mothers over 34, which is a reminder that age matters too.
  • Genetic counseling guidance (2002): first-cousin couples have an added risk of about 1.7 to 2.8 percentage points for significant birth defects, above the background risk every couple has.
  • Child mortality: a major review estimated mortality among children of first cousins about 3.5 percentage points higher than among children of unrelated parents, strongly shaped by social and economic factors.
  • India: analysis of national surveys linked consanguineous marriage to higher rates of miscarriage and newborn death, though not to deaths in later childhood.

Put simply: the risk is real and worth acting on, and most children of first cousins are born healthy.

When risk adds up over generations

The numbers above are for a single first-cousin marriage. Where cousin marriage has repeated over several generations, risk builds up. Double first cousins (when siblings from one family marry siblings from another) are often counseled at about three times the population risk. In families with generations of cousin marriage, couples who describe themselves as only distantly related can face a risk similar to first cousins.

What religion and law say

Cousin marriage is permitted in Islam, legal in Pakistan, the UK and most of Europe, and a long-standing custom in many communities. It's banned or restricted in some places, including many US states, China and the Philippines, and the Hindu Marriage Act prohibits first-cousin marriage unless an established custom allows it.

We don't take religious positions. This article is about health information, so couples can make their own informed decisions.

What cousin couples can do

  • Share your family history. Any known conditions in either family are important information.
  • Get a carrier screen together. Professional guidelines suggest that couples who are second cousins or closer consider broader screening panels. In a study of high-risk couples in Pakistan, 45% received a result that could change their decisions.
  • Test for thalassemia if your families come from South Asia, the Middle East or the Mediterranean. See how other countries approached it.
  • Talk to a genetic counselor before pregnancy, when you have the most options.

If you both turn out to carry the same condition, you still have choices: testing during pregnancy, IVF with embryo testing, or going ahead fully informed. The point is knowing, not judging.

Sources

  1. Bittles AH, Black ML. Consanguinity, human evolution, and complex diseases. PNAS (2010)
  2. Dawn: Consanguineous marriages in Pakistan (citing PDHS 2017–18)
  3. Consanguineous marriage in India, NFHS-5. Biodemography and Social Biology (2026)
  4. El-Mouzan MI et al. Consanguinity in Saudi Arabia. Saudi Medical Journal (2007)
  5. Sheridan E et al. Risk factors for congenital anomaly in a multiethnic birth cohort (Born in Bradford). The Lancet (2013)
  6. Risk of birth defects from cousin marriage revealed by Bradford study (PET BioNews)
  7. Small N et al. Changes in consanguineous marriage in Born in Bradford cohorts. Wellcome Open Research (2024)
  8. Bennett RL et al. Genetic counseling and screening of consanguineous couples and their offspring (NSGC). J Genet Couns (2002)
  9. First cousins face lower risk of having children with genetic conditions than is widely perceived (University of Washington, 2002)
  10. Hamamy H. Consanguineous marriages: preconception consultation in primary health care. J Community Genet (2012)
  11. Consanguinity and pregnancy outcomes in India (NFHS-4 and NFHS-5). Scientific Reports (2024)
  12. Gregg AR et al. Screening for autosomal recessive and X-linked conditions: ACMG practice resource. Genetics in Medicine (2021)
  13. Akbar et al. Expanded carrier screening in high-risk couples in Pakistan. Prenatal Diagnosis

Cousins? Get your personal answer.

A couples carrier check shows whether you two actually share a condition, instead of guessing from averages. Request it from any country.

Request a carrier check

Questions people ask.

What are the side effects of cousin marriage?

For first cousins, the main effect is a higher chance of children inheriting recessive genetic conditions: the risk of a serious birth defect roughly doubles, from about 3% to about 6%. Studies also link consanguinity to somewhat higher rates of miscarriage and newborn death. Most children of first cousins are healthy.

Is cousin marriage allowed in Islam?

Yes, cousin marriage is permitted in Islam. Whether to have genetic screening before marriage or pregnancy is a separate health decision that many couples make.

Does cousin marriage cause thalassemia?

No. Thalassemia is caused by inherited gene variants. Cousin marriage raises the chance that two carriers of the same variant have children together, which is why testing both partners matters.

Is second cousin marriage safe?

Second cousins share much less DNA than first cousins, and the added risk is small, under one percentage point above the background risk. A carrier screen can still check for shared conditions.

What tests should cousins take before marriage?

A carrier screen for both partners, a thalassemia test if your families come from a high-prevalence region, and a conversation with a genetic counselor before pregnancy.