Mutual compensation of the effects of religious and ethnic homogamy on reproduction.

OBJECTIVES Homogamy, mating based on similarity, has been demonstrated for a great variety of traits such as age, education, religion, and physical and psychological traits. Recently, pro-fertile effects of religious as well as educational homogamy have been reported. We investigate whether ethnic homogamy also has a pro-fertile effect and whether ethnic and religious homogamy interact in their putative effects on reproduction (in terms of average number of offspring). METHODS We analyzed the association between ethnic as well as religious homogamy and woman's average number of offspring based on census data from ten countries provided by IPUMS international, encompassing a total of 1,485,433 married women aged 46-60 years (who have thus completed or almost completed reproduction) and their spouses. RESULTS We find a clear pro-fertile but nonadditive effect of both ethnic and religious homogamy, which is most pronounced in the case of double homogamy. Our results further indicate that homogamy for one trait may compensate for heterogamy of the other, albeit countries differ regarding which trait compensates for the other. CONCLUSIONS We suggest that the interaction between ethnic homogamy, religious homogamy, and reproduction provides an interesting example for gene-culture co-evolution.