Human Fertility Increases with Marital Radius

Article excerpt

ABSTRACT

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.

MARITAL radius, i.e., the distance between mates' birthplaces, is a classic population structure parameter (Cavalli-Sforza and Bodmer 1971). According to Malécot's theory on spatial structure of genetic populations, the consanguinity coefficient of a mate can be well approximated by a decreasing function of the marital radius (Malécot 1959; Kimura and Weiss 1964; Morton 1969, 1977). We report here a positive association between marital radius and fertility, measured in a large population (all women born in Denmark in 1954). In light of Malécot's theory, this association provides strong evidence of the deleterious effect of low genetic mobility affecting an entire human population. Moreover, the possibility of spurious association due to effects of socioeconomic factors is discarded by an analysis using a range of explanatory variables representing key socioeconomic factors. This data will enable us to represent not only the effect of consanguinity but also the effect of local population size. The extension of the material used and the possibility of obtaining individual information on all the individuals in the populationmake our results a remarkable illustration of a classic theory of population genetics.

The study is based on the Danish Central Personal Register, a population register with almost perfect coverage. We constructed the cohort of all women born in Denmark in 1954 who were alive and living in Denmark in 1969, totaling 42,165 women. This cohort was followed up to the end of 1999, covering >1,200,000 persons/year. The number of children each mother had between the ages of 15 to 45 years old was determined and is called fertility. This period, termed the observation period, covered essentially the entire reproductive life of those women, since having children before the age of 15 or after the age of 45 is very rare in Denmark. A total of 22,298 women of the cohort had at least one child in the observation period. Note that an analogous study using a cohort formed by all men born in Denmark in 1954, although also of interest, would generate nontrivial issues due to censoring of the number of children each man had, because the fertility of men is not necessarily reduced after the age of 45.

The marital radius associated with each child born in the observation period to a mother in the cohort was estimated by the distance between the centroids of the parishes where the parents were born. Themarital radius of the mother in the cohort that had children in the observed period is defined as the mean marital radius associated with her children. Half of the women in the cohort who had children showed a marital radius <33 km. Using the links to the parents of all people born in Denmark in 1954 we discovered that the genetic mobility in the previous generation was lower; indeed, only 2.84% of the parents of children born in 1954 were born in different parishes. Therefore, the studied cohort originated from a population with a relatively low level of genetic mobility, which, however, presented variation in the level of consanguinity due to a partial increase in demographic and genetic mobility.

We present below strong evidence that fertility is positively associated with marital radius in the population in question. The Spearman correlation between the mean marital radius and the number of children was 0.038 (P-value to test for no correlation < 0.0001), indicating a general association between marital radius and fertility. The association was further characterized by fitting a truncated Poisson regression for predicting the number of children as a fourth degree polynomial function of the marital radius. …