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But why are some people more susceptible to developing those bad habits than others?

The spin on that article doesn't quite sit right with me (and note the editor has issued a correction, as well) but there are certainly some things there I'd agree with. Logically, ADHD must be a disorder neither of the individual nor of their environment, but of the interaction between the two that arises when the expectations of society don't match the behaviour of the individual. So whether you should try to 'fix' the individual or society is an ethical question not a scientific one. Practical considerations may lead to 'fixing' the individual with medication, but in some cases a change to their environment may be achievable to the same effect. e.g. an adult with mild to moderate ADHD symptoms may be lucky enough to get a job they're so interested in that they don't have any problems with focus.

I'm also guessing it's not helpful to frame these debates in a binary fashion. All psychological problems exist on a spectrum so there will always be a group in the middle for which e.g. diet or learned coping strategies work well enough. Then there will be a more extreme group for whom those tools fail. And a group in between those two who might struggle along for years looking like they're doing fine, but in the long term burn out in the absence of a comprehensive treatment.