ADD vs. ADHD: What’s the difference?

Despite ADD/ADHD being one of the most common neurodevelopmental disorders in childhood—around 11% of U.S. children age 4-17 have ever received a diagnosis of it, according to a 2011 CDC statistic—few people seem to know one really basic fact about it. So what the crap is the difference between ADD and ADHD?
Turns out, there really is no difference between the two, except for the fact that one of them technically isn’t a thing anymore. There are mentions of what appears to be the disorder as far back as 1798, and it seems no one could settle on a name or the criteria since; ADD and ADHD are just the most recent attempts to name and categorize it.
But which one is right, then?
ADD, or attention deficit disorder, was first recognized by that name in 1980 when it was added to the DSM-III (the third edition of the handbook used by healthcare professionals to diagnose mental disorders). The disorder had been in previous DSM versions, but had not been referred to as ADD. Further, in the DSM-III­, the criteria for diagnosis had been altered to strike out hyperactivity as a main criterion, noting that those with ADD did not always have hyperactivity, but rather had issues with attention and impulse control.
ADD became ADHD (attention deficit-hyperactivity disorder) only seven years later, in the revised version of the DSM-III. In this version—after more research was conducted on the matter—the criteria for diagnosis changed again. This time, the symptoms of inattention, impulsivity, and hyperactivity were combined into one list of possible symptoms, and diagnosis required a certain score from the list.
The name has remained the same since 1987, so technically referring to ADHD as ADD is incorrect. The most recent DSM (the fifth edition) has yet again a different diagnosis determination, involving having at least five or six inattention symptoms and/or five or six hyperactivity-impulsivity symptoms, depending on age, plus a set of other conditions. The full criteria list can be found here on the CDC’s website.
Based on these new criteria, there are three kinds of ADHD one can have: combined, meaning they have five or six each in the inattention and hyperactivity-impulsivity categories; predominantly inattentive, signifying a significant score in inattention but not hyperactivity-impulsivity; and predominantly hyperactive, meaning the opposite.
Often, ADHD fades away as children age, but an estimated 4.4% of adults have ADHD—which is associated with higher levels of unemployment and divorce, as well as drug and alcohol abuse and mental disorders like depression. These problems can be helped though; treatment for adults and children is very similar, including medication, psychotherapy, and treatment of concurrent mental health issues like depression.
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