Marriage between cousins, often referred to as consanguineous marriage, describes a union between individuals who share a common ancestor. This practice has a long history across various cultures and continues to be observed in different parts of the world today. While it might seem uncommon in some societies, these unions have been a part of human social structures for centuries, driven by a variety of factors.
Understanding Consanguineous Marriages
Consanguineous marriage specifically refers to a union between two people who are related by blood, meaning they share a common ancestor. In clinical genetics, this typically means individuals related as second cousins or closer.
Different degrees of cousin relationships exist, each reflecting a varying level of shared ancestry. First cousins share a set of grandparents, meaning their progeny are homozygous at approximately 1/16 of all loci. Second cousins, on the other hand, share a set of great-grandparents. Beyond second cousins, the genetic influence from shared ancestry generally becomes less significant, often comparable to that seen in the general population.
Global Practices and Historical Roots
Cousin marriages have been widely practiced throughout human history and across diverse cultures and regions. Historically, these unions were common in royal families to maintain power, secure political alliances, and preserve perceived “purity of bloodlines”. For instance, the Habsburg dynasty frequently intermarried to retain power, leading to the transmission of genetic abnormalities.
Beyond royalty, various social, economic, and traditional reasons have supported cousin marriages. In many societies, particularly in the Middle East, South Asia, and North Africa, these marriages are still prevalent. Reasons include maintaining family honor, preserving wealth and property within the family, strengthening familial ties, and facilitating easier marital arrangements. Approximately 8.5% of children worldwide have consanguineous parents.
Legal Status Across Regions
The legal status of cousin marriages varies significantly across the globe and even within countries like the United States. In many European countries, cousin marriage is legal, although it is generally uncommon and not widely practiced in mainstream culture. In contrast, some countries, such as China, North Korea, South Korea, and the Philippines, legally prohibit first-cousin marriages.
Within the United States, there is no overarching federal law governing cousin marriages; legality depends on state law. As of February 2025, 18 U.S. states permit marriages between first cousins, while 24 states prohibit them. Additionally, eight states allow first-cousin marriages under specific conditions, such as requiring genetic counseling or if both individuals are beyond reproductive age.
Genetic Implications
Consanguineous marriages carry genetic implications due to the increased likelihood of shared genetic material between parents. When two individuals who are closely related have children, there is a higher chance that both parents carry the same recessive gene mutations, which can then be passed on to their offspring. This increased genetic similarity leads to a higher probability of homozygosity, where an individual inherits two identical copies of a gene, one from each parent.
Recessive genetic disorders manifest only when an individual inherits two copies of a mutated gene, one from each parent. While carriers of a single mutated gene are typically healthy, they can pass the mutation to their children. In consanguineous unions, the probability of both parents being carriers for the same recessive mutation is significantly elevated compared to unions between unrelated individuals. This means children of first-cousin marriages have a 4-6% risk of autosomal recessive genetic disorders, compared to a 3% risk in children of unrelated parents.
Societal Perspectives and Individual Choice
Societal perspectives on cousin marriages are diverse, shaped by cultural, historical, and regional contexts. In some communities, particularly in parts of the Middle East and South Asia, cousin marriage is viewed favorably and can even be an expected practice. These unions are often seen as a means to enhance family unity, maintain wealth within the family, and potentially lead to greater compatibility between spouses due to shared social backgrounds. From an individual perspective, marrying a cousin can offer social support and reinforce existing family networks.
However, in many Western societies, cousin marriage is less common and may face social stigma or even legal prohibition. The perception of increased genetic risks for offspring has contributed to changing social norms regarding these unions. Balancing individual autonomy and the desire to uphold cultural traditions with modern health considerations, particularly regarding genetic risks, remains a complex aspect of these marriages.