Ultimately the "environmental" factor depends on how much the environment naturally varies from family to family. Anyway, having one IMO kid is virtually not p

Ultimately the "environmental" factor depends on how much the environment naturally varies from family to family. Anyway, having one IMO kid is virtually not possible with most parents' genetic contribution, and there are plenty of IMO competitors with siblings that would have no hope of being such, where, if there was an environmental factor, the question was decided by the second birthday as a result of air quality, lead paint, or what music was on the radio.