For maximum fertility, marry more than 20 km from your birthplace

Labouriau and Amorim (2008) show that women have more children if they were born farther from their husbands:

We report a positive association between marital radius (distance between mates' birthplaces) and fertility detected in a large population. Spurious association due to socioeconomic factors is discarded by a conditional analysis involving income, education, and urbanicity. Strong evidence of consanguinity's deleterious effects affecting an entire human population is provided.

They followed a birth cohort of women in Denmark, all born in 1954.

This result may be qualitatively similar to the third and fourth cousin marriage paper from earlier this month. In that case, Iceland couples with the maximum fertility were those between third and fourth cousins -- both closer and further consanguinous matings resulted in fewer offspring on average.

In this study, couples born further apart had more children. The distance between the woman's and man's birthplaces is called the "marital radius." The data from the paper show that the relationship between marital radius and fertility is mostly explained by a reduced fertility for couples with marital radius less than 15 km.

Figure 1 from Labouriau and Amorim, 2008.

The result cannot be explained by a small proportion of high-reproducing couples, as demonstrated by the percentage of couples with more than two children, which tracks the average number of children. There is a slight negative correlation between urban residence and fertility; the authors show by partial correlation that this alone does not explain the reduced fertility with low marital radius (which is important, since a large fraction of the low-marital-radius couples may have been born in the city).

One potentially important variable was not included: marital age. To be sure, some women who marry men at low marital radius may be swept away as teenagers and go on to have eight children. But others may be settling, marrying late in life.

This study isn't orthogonal to the Iceland cousin study, but it adds another element. These people are about as close to the genetics of Icelanders as we can hope to get. The authors suggest that greater mobility in the last 50 years has removed a significant inbreeding depression. Yet, the inbreeding between people born within 15 km of each other is mostly at the level of third cousins or further -- the sample that had the highest fertility in Iceland. Curious.

It would be interesting to see whether this result holds over longer distances. levels out, or even reverses.

References:

Labouriau R, Amorim A. 2008. Human fertility increases with marital radius. Genetics 178:601-603. doi:10.1534/genetics.107.072454