A union between first cousins, who share a set of grandparents, is known as consanguinity. Understanding the implications requires looking at the genetic mechanisms involved, quantifying potential health outcomes, navigating the laws that govern such marriages, and exploring available medical options. The risks, acceptance, and legal status vary significantly depending on geography and family history.
The Basics of Shared DNA
Every person inherits half of their genetic material from each parent. Since first cousins share a pair of grandparents, they share approximately 12.5% of their genetic information inherited from those common ancestors. This shared ancestry forms the biological basis for the slightly elevated risk in their offspring.
The potential for health concerns stems from the nature of recessive genes. For most genetic conditions, a person must inherit two copies of a mutated gene—one from each parent—to develop the disorder. If a person inherits only one copy, they are typically a healthy carrier. Two unrelated people have a low probability of carrying a mutation for the same rare disorder.
However, when parents are related, the chance that they both carry the same rare recessive gene is significantly higher, as they may have both inherited it from their shared grandparents. If both parents pass on that single, non-working copy, their child will inherit two copies and be affected by the genetic condition. This mechanism explains why children born to related parents have a modestly increased likelihood of inheriting an autosomal recessive disorder.
Statistical Increase in Health Concerns
The primary concern for children born to first cousins involves the increased statistical probability of developing a severe genetic condition. In the general population, the baseline risk for a child to be born with a significant birth defect or severe genetic disorder is estimated to be between 2% and 3%.
For the offspring of first-cousin unions, this overall risk is elevated, typically rising to a total risk of between 4% and 6%. This represents an additional risk of approximately 1.7% to 2.8% above the background risk of the general population. While this increase is noticeable, the vast majority of children born to first-cousin couples will be perfectly healthy.
The disorders that account for this statistical increase are primarily autosomal recessive conditions. Examples of these conditions include cystic fibrosis, spinal muscular atrophy, and certain metabolic disorders. In some communities where consanguineous marriage has been practiced for many generations, specific conditions like beta-thalassemia or congenital heart defects may show a higher incidence due to the concentration of certain recessive genes within the family line.
The Varying Legality of Cousin Marriage
The legal status of first-cousin marriage shows considerable variation, especially within the United States. Laws are determined at the state level, creating a complex legal landscape. Currently, about half of US states prohibit marriage between first cousins, while the remaining states either permit it outright or allow it with specific requirements.
These requirements can include mandatory genetic counseling before marriage or proof that one or both partners are unable to reproduce. The legal prohibition in the US is rooted primarily in historical concerns about the health of offspring. In states where the practice is illegal, some recognize marriages performed legally in other jurisdictions, while others explicitly void all cousin marriages, even if they were conducted elsewhere.
Globally, the cultural and legal landscape is markedly different. Marriages between first cousins are common and often culturally preferred in many parts of the world, particularly in the Middle East, North Africa, and South Asia. In some regions, over 20% of all marriages are consanguineous, with the practice being seen as a way to strengthen family ties, preserve property, or simplify the marriage arrangement.
Options for Genetic Screening
Couples who are first cousins and are planning to have children have access to medical resources that can provide a more precise assessment of their specific genetic risk. The most important first step is preconception genetic counseling with a certified professional. This involves taking a detailed family history, or pedigree, to identify any known genetic conditions that run in either family line.
This consultation allows the counselor to determine the couple’s unique risk profile, which may be higher or lower than the general statistical average depending on their ancestry. Actionable steps include modern carrier screening panels, which can test both partners simultaneously for carrier status for hundreds of severe autosomal recessive disorders. If both partners are found to carry a mutation for the same condition, they can be informed of their 25% risk for an affected child.
Identifying a shared mutation allows the couple to make informed reproductive choices, such as pursuing prenatal diagnosis, preimplantation genetic diagnosis with in vitro fertilization, or using donor gametes. For couples without any concerning family history, genetic counseling provides reassurance and the option to proceed with expanded carrier screening.