Does being an only child, versus having siblings, affect ADHD risk?

There's no strong evidence for a meaningful link either way. Sibling status doesn't appear among the established risk factors for ADHD in the research literature, which points overwhelmingly toward genetics as the dominant factor, alongside a smaller set of specific environmental and prenatal risk factors like premature birth or certain toxin exposures. Where family structure sometimes gets studied is in relation to how symptoms are noticed or reported, an only child's behaviour has no sibling comparison point, which could affect when a parent first raises a concern, but that's a detection factor, not evidence that sibling status changes actual ADHD risk. References: https://www.caddra.ca/wp-content/uploads/Canadian-ADHD-Practice-Guidelines-4.1-January-6-2021.pdf

I'm an only child with ADHD and my cousin, who has three siblings, also has it. Never seen any real pattern there beyond coincidence in my own family. Nobody in our extended family shares it either, for whatever that anecdote is worth. Feels like one of those theories that sounds tidy but doesn't hold up once you actually check it against real families.