What is 90% of ADHD caused by?

Quick Answer:

Genetics - mostly. Twin and family studies put ADHD's heritability at roughly 74–88%, often rounded up to 'about 80–90%' in casual conversation, making it one of the most heritable conditions in psychiatry, right up there with autism and schizophrenia. The rest gets shaped by environmental factors like premature birth, low birth weight, or prenatal exposures - but no, it's not caused by screens, sugar, or bad parenting, whatever your aunt says at dinner.