I think they usually _don’t_ use twin studies because they are harder/more expensive. In this case they used the Philadelphia Neurodevelopmental Cohort: https:

I think they usually _don’t_ use twin studies because they are harder/more expensive. In this case they used the Philadelphia Neurodevelopmental Cohort: https://www.med.upenn.edu/bbl/philadelphianeurodevelopmentalcohort.html https://www.med.upenn.edu/bbl/philadelphianeurodevelopmental... I don’t see any mention of twins here or on JAMA, although I can’t access the full text currently.