> How is it that children who are very closely matched on a great many environmental and familial variables often differ profoundly in various attributes of aca

> How is it that children who are very closely matched on a great many environmental and familial variables often differ profoundly in various attributes of academic ability and personality? The simple answer is that the children's circumstances are not matched. Fundamentally, a first child is going to have a different experience from the second child, on account of how the first child has no sibling when they're developing. Go deeper and you'll find more unaccounted-for variables: parents' childrearing experience, amount of leftover budget for childcare/food, macroeconomic and political conditions, or even something like water quality.