A recent study look at diagnostic reliability of ADHD:
RESULTS: Many mental health practitioners seem to proceed heuristically and base their decisions on unclear rules of thumb. The respondents more readily diagnosed ADHD when the case involved a male patient and presented prototypical symptoms, such as impulsiveness, motoric restlessness, and lack of concentration. They were, for instance, twice more likely to conclude ADHD with the boy version of the vignettes than with the girl vignettes. Interestingly, even the therapist’s gender played a role in the diagnostic as male doctors diagnosed ADHD more frequently than their female counterparts.
CONCLUSION: ADHD is over-diagnosed because doctors rely too much on their intuition and not on defined, established diagnostic criteria.