Third cousin couples have the most children and grandchildren

Marriage between closely related cousins is a heavy taboo in many cultures and its critics often cite the higher risk of genetic diseases associated with inbreeding. That risk is certainly apparent for very close relatives, but a new study from Iceland shows that very distant relatives don’t have it easy either. In the long run, they have just as few children and grandchildren as closely related ones.

Shuffling the genetic deck

Sex chromosomes aside, every person has two copies of each gene, one inherited from their father and one by their mother. Not every gene will be in correct working order, but there’s a good chance that a faulty copy will be offset by a functional one from the other parent.

However, if two parents are closely related, there’s a higher-than-average chance that they will already share some of the same genes and a similarly increased chance that their child will receive two defective copies. That can be very bad news indeed and in cases where important genes are affected, the results can include miscarriage, birth defects or early death.

Sex, then, is a shuffling of their genetic deck and theoretically the more closely related the partners are, the greater the chance that their child will be dealt a dud hand. And yet, some studies have found that some closely related couples actually do better than distant relatives in terms of the number of children they manage to raise. This trend is certainly unexpected and the big question is whether it is the result of biology or money.

Wealth or genes

In societies where close relatives marry, these unions tend to happen at a relatively early age and they provide avenues for families to retain wealth and land within bloodlines. These related couples enjoy the health benefits enjoyed by the rich as well as more time in which to raise a larger family. Together, these two effects could more than make up for any disadvantages wrought by their genes.

Earlier studies have done little to clear the confusion. They have mostly been conducted in parts of the world like India, Pakistan and the Middle East where marriage between close relatives is relatively frequent, but which are also home to enormous gulfs between the richest and poorest members of society. With demographics like these, sorting out the relative contribution of socioeconomics and biology is difficult.

To do that, what you need is a country with a small population where couples are reasonably closely related and with a very shallow gradient between rich and poor. Ideally, you’d also want this country to have excellent family records dating back several years. In short, you’d want to base your study in a country almost exactly like Iceland.

200 years of Iceland

Iceland is home to a tiny population of just over 300,000 people who enjoy a level of social equality that is almost unparalleled elsewhere in the world. Wealth, family size and cultural practices are fairly uniform. The country is also home to uniquely impressive geneaological records that allow today’s Icelanders to track their family trees with exacting precision for centuries. These records are supplemented by thorough medical records and thousands of willingly donated genetic samples.

Agnar Helgason from deCODE Genetics, a pharmaceutical company located in Reykjavik, made good use of these records to study over 160,000 Icelandic couples since 1800. At this time, Iceland was still a poor agricultural nation and close-knit rural communities meant that on average, couples were related at the level of third or fourth cousins.

Since then, the country has prospered into a wealthy industrial one and the growing population has shifted to a mainly urban way of life. In doing so, people became more likely to find partners who were more distantly related and by 1965, couples were only related at the level of fifth cousins on average

As expected, Helgarson’s study unveiled the dangers of close inbreeding. While the most closely related couples had the highest number of children, many of them failed to live long enough to have children of their own and in the long run these couples had the fewest grandchildren.

But surprisingly, distantly related couples were at a disadvantage too. In fact, Helgarson found that couples related at the level of third cousins eventually fostered the largest families. For example, among women born between 1800 and 1824, those partnered with men who were third cousins had an average of 4 children and 9 grandchildren, while those partnered with a distant eighth cousin had just 3 children and 7 grandchildren. For starting large families, very distant relatives were just as poor prospects as very close ones.

Over the 200 years included in the study, Iceland has seen a steep decline in both fertility and relatedness between couples. And despite all that, for every 25-year period the Helgarson looked at, the same pattern held – couples who were moderately closely related ended up with the largest number of descendants.

Implications

These remarkably consistent results have convinced Helgarson that the counterintuitive effect must have some biological foundation. Its exact nature will have to wait for another study and for now, we are left only with speculation.

It could be that a child’s immune system may be more compatible with its mother’s if its father is reasonably closely related to her. Alternatively, a union between distant relatives could serve to splinter groups of beneficial genes that have evolved in close association with each other.

The study’s implications for societal taboos against marriages between close cousins is open for debate. Certainly, it doesn’t mean that singletons should be sifting through their phone books on the hunt for attractive third cousins. However, the relatively poor reproductive success of distant relatives has the potential to explain the massive decline in fertility in many countries the world over.

In the time that Iceland has gone from rural agriculture to urban industry, its population growth has slowed and its fertility rates have declined, a trend shared by a slew of other nations. Helgarson suggests that this could, at least in part, be due to people finding ever more distantly related partners.

Image: Wedding photo by Claude Renault

Reference: Helgason, A., Palsson, S., Guthbjartsson, D.F., Kristjansson, t., Stefansson, K. (2008). An Association Between the Kinship and Fertility of Human Couples. Science, 319(5864), 813-816. DOI: 10.1126/science.1150232