Distant relatives who married were found to have more children

The Science journal study found third and fourth cousin couples had more children than those more distantly related.

These cousins may be biologically more compatible, the specialist genetics firm deCODE concluded.

However, there was no advantage in partnerships involving first cousins.

Our results show that couples related at the degree of third to fourth cousins exhibited the greatest reproductive success

deCODE researchers

Evidence that related couples have more children has been found before, but there has always been doubt whether this is the result of similarities in their genetic makeup, or just the result of differences in the societies in which cousin marriages are common.

The deCODE team have access to a unique genetic record which allows them to unpick the interlocking family trees of Iceland.

In addition, Icelandic society and culture has been historically uniform across its population, making it easier to rule it out as an influence on the results.

The study was able to detect a distinct pattern of childbearing.

Between 1800 and 1824, for example, third cousin couples - those who share great-great grandparents - had on average 4.04 children and 9.17 grandchildren, compared with 3.34 and 7.31 for couples composed of eighth or higher cousins.

A similar ratio for couples between 1924 and 1949 was spotted.

Fourth cousin couples - those sharing a great-great-great grandparent - were also likely to have more children, although more closely related cousins were not.

City life

This, said the researchers, perhaps reflected the genetic disadvantages of trying to have children with someone more closely connected.

A child will get two copies of every gene - one from each parent - and if they have one copy of a mutated gene that could harm their development, normally the other copy, from the other parent, is normal.

Close relatives, however, are more likely to each pass on the same disadvantageous gene.

The researchers wrote that the shift from smaller, rural communities to large cities - a feature throughout the modern world, was "a new situation for humans in evolutionary terms".

If "kinship" did offer an advantage, they said, then urban living, which reduced the chances of close cousins becoming partners, could slow the birth rate.

Laurence Shaw, a fertility specialist from the Bridge Centre in London, said that the research again raised the question of whether cousins found it easier to conceive because their immune systems were more "tolerant" of each others.

He added: "The other thing I wonder is whether the limit of 10 set by the Human Fertilisation and Embryology Authority as to the number of times a single sperm donor can be used could be re-examined in the light of this."