More recently, Lummaa and her colleagues have been studying how sons are not just tough on their mothers but also hard on their siblings. Those born after a son were physically slighter, had smaller families and generally had a greater chance of dying from an infectious disease. The effects held up whether the elder brother died in childhood or not, suggesting that the negative outcome is not a result of some direct sibling interaction, such as competition for food, regular beatings or the practice of primogeniture, in which the eldest brother inherits everything. “Big brothers are bad for you,” Lummaa explains. “If the fifth-born was a male, then the sixth-born is doing worse.” [More]

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