A study of ten million American siblings has found that the order in which a child is born into a family carries measurable associations with the diseases they are likely to develop across a lifetime. First-borns appear more prone to neurodevelopmental and allergic conditions, while second-borns show elevated rates of substance abuse and gastrointestinal disorders — patterns that held even when researchers controlled for shared genetics and family circumstance. The findings, drawn from two decades of insurance claims, do not establish cause, but they invite a deeper reckoning with how the sequ