Table 1.

A comparison of the characteristics of ADHD and ADD

ADHD With Hyperactivitya	ADDb
Hyperactive, always on the go, impulsive	A significant subset are hypoactive and sluggish and have slow response speeds.
Primary deficit in response inhibition	Primary deficit in working memory, especially prominent in auditory processing because of the demands it places on working memory
Often insufficiently self-conscious	Tend to be overely self-conscious
Social problems because too assertive and impulsive: butt in, take things belonging to others, fail to wait their turn, and act without first considering the feelings of others	Social problems because too passive, shy, or withdrawn
Tend to be extroverted	More likely to be introverted
Externalizing behaviors, such as conduct disorder, aggressivity, disruptive behavior, and even oppositional defiant disorder are far more commonly comorbid with ADHD than with ADD.	Internalizing disorders, such as anxiety or depression, are somewhat more common in children with ADD than those with ADHD. ADD children tend to be socially isolated or withdrawn. Reading and language deficits and problems with mental mathematical calculations are more commonly comorbid with ADD than with ADHD.
Respond positively to methylphenidate (Ritalin)	A significant percentage are not helped by methylphenidate.
Most respond positively to methylphenidate in moderate to high doses.	Those who are helped by methylphenidate often do best at low doses.
Those with ADHD are more likely to smoke than are those with ADD. (There are marked similarities in the neurobiological and psychological effects of nicotine and methylphenidate.)	A significant subset are helped by amphetamines rather than methylphenidate. Amphetamines affect both the reuptake and release of catecholamines. A marked deficit in the release of DA and NE might cause sluggishness and underarousal.
Methylphenidate adresses catecholamine reuptake. Addressing reuptake appears to be sufficient to help individuals with ADHD.	People with ADD are not so much easily distracted as easily bored. Their problem lies more in motivation than in inhibition.
	Challenge or risk, something to literally get their adrenaline pumping, can be key to keeping their attention and to eliciting optimum performance. Individuals with ADD, although typically shy, may engage in risk-taking and thrill-seeking activities as ways to experience a level of engagement they have difficulty sustaining in their daily lives.
Converging evidence for a primary disturbance in the striatum	A primary disturbance in prefrontal cortex is implicated.
The primary neural circuit that is affected may be a frontal–striatal one.	The primary neural circuit that is affected may be a frontal–parietal one.
Polymorphisms in the DAT1 gene are associated with ADHD. This is consistent with the centrality of the striatum in ADHD because DAT plays a particularly important role there. It is also consistent with the efficacy of methylphenidate because DAT is the primary target for the clinical action of methylphenidate.	The 7-repeat allele polymorphism of the DRD4 gene is more strongly linked to ADD than to ADHD. This is consistent with the centrality of prefrontal cortex in ADD because the D4 DA receptor is present in prefrontal cortex but not in the striatum in humans.

^aChildren with combined-type ADHD or hyperactive-type ADHD.

^bChildren with inattentive-type ADHD, excluding those with significant hyperactivity, even if they fail to meet the criteria on seven hyperactivity items (they are really combined-type ADHD).