On the Question of Cousin Marriage — Science, Rights, and Misconceptions

Sacred matrimony

Even in today’s so-called civilised societies, cousin marriage still evokes unpleasant remarks as if they were perfectly justifiable — like it were somehow acceptable to cringe at these lawfully wedded spouses. And wow, now we've suddenly started to hear of plans in some countries to ban cousin wedlock altogether.

This would happen on supposed genetic grounds, since clear problems have been shown to arise only in closed communities where cousin unions recur generation after generation, reducing genetic diversity — an entirely different phenomenon.

The now widely discussed "Bradford study" is exactly about that. It examined one small, socially and genetically isolated community in Northern England, where cousin marriages had been perpetually repeated. Its findings are not applicable to the wider population, nor can they be used to claim genetic risk in ordinary cousin unions. 

Common sense tells us that if the risk of anomalies had truly been significant, humankind would have been genetically devastated ages ago! After all, we are talking about humanity’s largest and longest-running natural experiments here: for thousands of years, across every continent, culture, and religion, first cousins have married and had children, yet no wave of congenital disorders has ever appeared.

There is an enormous amount of information on the risks openly available to whoever is willing to look, and within it, no data that supports any alarm — not today, nor at any point in medical history. Even Charles Darwin himself concluded that a single cousin marriage carries no meaningful genetic threat. Contemporary population genetics broadly supports this view, and international health policy aligns accordingly.

The World Health Organization remains the authority on this issue. Its guidelines are based on extensive scientific literature, meta-analyses across multiple populations, and broad global consensus.

I have found no WHO or other international recommendations to prohibit cousin marriage. Their materials emphasize genetic counselling, informed choice, and education — just as they do for any other couple.

You see, all couples carry a baseline risk of up to 3% for having a child with a congenital condition or developmental disability.* The risk for first-cousin couples (4–6%) is actually similar to that for a 34-year-old mother (4–5%). While some give birth even at a much older age, it's crucial to note that the risk for a 40-year-old mother is 6–8% and for a 50-year-old mother 10–15%. 

So whose children, then, have the “right” to be born?

To impose bans without factual evidence would represent a return to eugenic reasoning — something that modern societies have long condemned.

Cousins are not in a direct line of descent, and therefore cousin marriage is legal in most countries worldwide. (Click map below!) If it were truly harmful, civil laws would prohibit it — and these couples would never have been granted marriage licences in the first place.

Legislation and judgement must follow sound science and human rights. To deny legal recognition or dignity to cousin couples — or to stigmatize their children! — is to abandon reason in favour of prejudice and discrimination.

Map first-cousin marriage legality in dark blue!

Disclaimer: Due to the usual assumptions made about social media posts — no, my husband and I are not first cousins. 

*References (as provided by AI upon request):

World Health Organization (WHO). Congenital Disorders: Key Facts and Global Health Estimates. Geneva: WHO.

Centers for Disease Control and Prevention (CDC). Data and Statistics on Birth Defects. U.S. Department of Health and Human Services.

Morris, J. K., et al. (2002). "The risks of chromosomal abnormalities in the offspring of older mothers." BMJ (British Medical Journal), 324(7334), 809–812.

Hook, E. B. & Cross, P. K. (1982). "Rates of Down syndrome related to maternal age." American Journal of Human Genetics, 34(3), 365–373.