Birth order and face-to-face contact with a sibling: Firstborns have more contact than laterborns

The evolutionary theory of kin selection predicts that individuals may invest time and resources in their siblings, but that older siblings will invest in younger ones more than vice versa. This leads us to predict that firstborns are more likely to keep in touch with their sibling(s) than middleborns or laterborns. Using a large-scale dataset from the Netherlands (n1 = 1558), firstborns were indeed found to have significantly more frequent face-to-face contact, on a weekly basis, with a sibling than middle- or lastborns. This effect was found using multinomial logistic regression in which we controlled for other factors (educational attainment, difference between siblings in educational attainment, age and gender). The finding that firstborns are significantly more likely to keep in touch with their sibling on a weekly basis than laterborns remained unaltered after controlling for geographical distance between siblings (n2 = 1394). Middleborns did not differ significantly from lastborns in contact with their sibling(s). Findings are discussed with reference to research on birth order and family relationships.