Hyperfocus is something in between a rumor and a symptom. Among people with ADHD, it’s a commonly recognized phenomenon. Anecdotally, many people with ADHD can’t concentrate on some things but concentrate “too much” on others. Still, it’s a stretch to say hyperfocus is a straight-up symptom of ADHD because there’s no mention of it in diagnostic guides and surprisingly little research has been done on it.