The association of birth order with later body mass index and blood pressure: a comparison between prospective cohort studies from the United Kingdom and Brazil

Published online 29 October 2013Previous studies have found greater adiposity and cardiovascular risk in first born children. The causality of this association is not clear. Examining the association in diverse populations may lead to improved insight.We examine the association between birth order and body mass index (BMI), systolic and diastolic blood pressure (SBP/DBP) in the 2004 Pelotas cohort from southern Brazil and the Avon Longitudinal Study of Parents and Children (ALSPAC) from Bristol, south-west England, restricting analysis to families with two children in order to remove confounding by family size.No consistent differences in BMI, SBP or DBP were observed comparing first and second born children. Within the Pelotas 2004 cohort, first born females were thinner, with lower SBP and DBP; for example, mean difference in SBP comparing first with second born was -0.979 (95% confidence interval -2.901 to 0.943). In ALSPAC, first born females had higher BMI, SBP and DBP. In both cohorts, associations tended to be in the opposite direction in males, although no statistical evidence for gender interactions was found.The findings do not support an association between birth order and BMI or blood pressure. Differences to previous studies may be explained by differences in populations and/or confounding by family size in previous studies.L D Howe, P C Hallal, A Matijasevich, J C Wells, I S Santos, A J D Barros, D A Lawlor, C G Victora and G D Smit

Are elder siblings helpers or competitors? Antagonistic fitness effects of sibling interactions in humans

Determining the fitness consequences of sibling interactions is pivotal for understanding the evolution of family living, but studies investigating them across lifetime are lacking. We used a large demographic dataset on preindustrial humans from Finland to study the effect of elder siblings on key life-history traits. The presence of elder siblings improved the chances of younger siblings surviving to sexual maturity, suggesting that despite a competition for parental resources, they may help rearing their younger siblings. After reaching sexual maturity however, same-sex elder siblings' presence was associated with reduced reproductive success in the focal individual, indicating the existence of competition among same-sex siblings. Overall, lifetime fitness was reduced by same-sex elder siblings' presence and increased by opposite-sex elder siblings' presence. Our study shows opposite effects of sibling interactions depending on the life-history stage, and highlights the need for using long-term fitness measures to understand the selection pressures acting on sibling interactions

Towards a methodology for integrated history and philosophy of science

We respond to two kinds of skepticism about integrated history and philosophy of science: foundational and methodological. Foundational skeptics doubt that the history and the philosophy of science have much to gain from each other in principle. We therefore discuss some of the unique rewards of work at the intersection of the two disciplines. By contrast, methodological skeptics already believe that the two disciplines should be related to each other, but they doubt that this can be done successfully. Their worries are captured by the so-called dilemma of case studies: On one horn of the dilemma, we begin our integrative enterprise with philosophy and proceed from there to history, in which case we may well be selecting our historical cases so as to fit our preconceived philosophical theses. On the other horn, we begin with history and proceed to philosophical reflection, in which case we are prone to unwarranted generalization from particulars. Against worries about selection bias, we argue that we routinely need to make explicit the criteria for choosing particular historical cases to investigate particular philosophical theses. It then becomes possible to ask whether or not the selection criteria were biased. Against worries about unwarranted generalization, we stress the iterative nature of the process by which historical data and philosophical concepts are brought into alignment. The skeptics' doubts are fueled by an outdated model of outright confirmation vs. outright falsification of philosophical concepts. A more appropriate model is one of stepwise and piecemeal improvement