Why can ADHD, a disorder of attention, also come with intense focus?

Because ADHD is not a lack of attention but a difficulty in directing it, an executive function of the frontal lobe. Psychiatrically, inattention and an intense, locked-on concentration called hyperfocus are both considered traits of ADHD. So the same person can struggle to keep their attention on one task and struggle to pull it away from another. Hyperfocus often switches on when something is very interesting or gives instant gratification, such as computer games or online chatting, and concentration may then last for hours on end. The catch is that it can side-track a person away from assigned or important tasks.

Why attention locks on like this isn't settled. In ADHD, hyperfocus may be a coping mechanism or a symptom of emotional self-regulation. Another proposal links it to brain lateralization: people prone to hyperfocus, such as those with ADHD, may experience a 'pseudoneglect' in which attention is dominant on one side of the brain. That idea is still under study and not yet empirically proven. What is recognized is the pairing itself: the 2019 Updated European Consensus Statement on Adult ADHD describes inattentive symptoms occurring together with what it termed hyperfocus.

Hyperfocus isn't unique to ADHD. It has been proposed as a trait of autism spectrum disorder, where one suggested explanation is that the focused-on activity is predictable, and of schizophrenia, where it is hypothesized to be an intense focus on the information in front of a person that makes it hard to spread attention across several things.

It can look a lot like 'flow', the deep absorption people tend to praise. Both can help someone achieve things in some circumstances and become a liability in others, distracting from the task at hand. But flow is often described in more positive terms, which suggests the two are not simply two sides of the same condition.