ADHD, or attention deficit hyperactive disorder, has been long diagnosed as a psychiatric disorder. It's found commonly in children who were born from a mother who drank or smoked while pregnant, but this isn't the only cause. In fact, some cases of the disorder are purely genetic.
On the other hand, is it really a bad thing to have? - Those with ADHD typically have a lower count of dopamine receptors, and this means rewards have less impact on concentration.
Nonetheless, it's thought that ADHD may be a trait that could have been useful in the hunter-gatherer times, in which those with the disorder would be less likely to come home empty-handed because of their restless and hyperactivity.
What's more is those with these overactive minds are more likely to think outside the box and have more creativity. This stems from random thought processes that many others don't have. This kind of ability is useful in some careers where constantly-working minds are useful.