Why you don’t want an older brother or a younger pair of twin siblings:

Got an older brother? You’re less likely to have kids:

Sex-specific sibling interactions are potentially important in human ecology. It is well established that in patrilineal societies that sons suffer from the presence of brothers because of competition for inheritance. However, offspring (of both sexes) might also suffer from being born after an elder brother because of the greater costs sons may entail for their mother. Evidence that the cost of producing sons is higher has been gained from studies of ungulates and humans, with some of this cost being manifested as lower birthweight or reproductive performance of offspring born following a male. Using church record data from preindustrial Finland, we shed light on this process by investigating the demographic ‘mechanisms’ by which offspring born following an elder brother are compromised. First, we show that, for both men and women in this population, being born after an elder male sibling is associated with reduced probability of reproducing, a later age at first reproduction, and longer interbirth intervals. Second, we show that the primary effect of interest is a reduced probability of reproducing in those born after an elder brother (even among only those who married). Finally, we show that the total number of elder brothers who survived to adulthood has a negative effect on male offspring only, and this effect is independent of the elder brother effect above. We highlight that differences in the success of human offspring are not always social in origin as is often perceived but can also be biological, resulting from differential costs for mothers of producing male versus female offspring.

Source: “Elder brothers affect the life history of younger siblings in preindustrial humans: social consequence or biological cost?”

And what about twin siblings? Even worse: they’ll make you dumber:

How do families influence the ability of children? Cognitive skills have been shown to be a strong predictor of educational attainment and future labor market success; as a result, understanding the determinants of cognitive skills can lead to a better understanding of children’s long run outcomes. This paper uses a large dataset on the male population of Norway and focuses on one family characteristic: the effect of family size on IQ. Because of the endogeneity of family size, we instrument for family size using twin births and sex composition. IV estimates using sex composition as an instrument show no negative effect of family size; however, IV estimates using twins imply that family size has a negative effect on IQ. Our results suggest that effect of family size depends on the type of family size intervention. We conclude that there are no important negative effects of expected increases in family size on IQ but that unexpected shocks to family size resulting from twin births have negative effects on the IQ of existing children.

Source: “Small Family, Smart Family? Family Size and IQ Scores of Young Men” from the Centre for Economic Policy Research