Ecological Constraints on Marriage in Rural Ireland

Abstract

Behavioral ecological studies of cooperatively breeding birds suggest that delayed dispersal and reproduction are caused by ecological constraints on independent breeding opportunities. Here we use census data on marriage and reproduction among the 19th and 20th century rural Irish to determine if the ecological constraints hypothesis can be extended to humans and what modifications might be required. We focus specifically on the following predictions: (1) marriage rates for farmers varied directly with the availability of farms; (2) the incidence of celibacy among male heirs increased as size of farm decreased; (3) emigration increased as economic opportunities in rural Ireland decreased; (4) emigration rates were inversely related to farm size; and (5) emigrants improved their chances for marriage by leaving Ireland. Despite important differences between humans and other species, we conclude that the rural Irish fall within the scope of ecological constraints theory. Unmarried siblings who remained on the home farm potentially gained some indirect fitness benefits because (1) the labor of unmarried siblings probably enhanced farm wealth; and (2) heirs of wealthier farms had higher reproductive success. The latter prediction implies that increased wealth, whether due to siblings or other causes, was reproductively valuable.