Table 1.

Comparison of the Characteristics of ADHD (Attention Deficity Hyperactivity Disorder) and ADD (Attention Deficit Disorder)

ADHD that includes hyperactivity (i.e., children with ADHD-combined-type and ADHD-hyperactive)	ADD (i.e., children with ADHD-inattentive excluding those with significant hyperactivity even if they fail to meet criterion on 7 hyperactivity items [those children are really ADHD-combined-type])
• Hyperactive, always on the go, impulsive	• A significant subset are hypoactive, sluggish, with quite 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 overly self-conscious
• Social problems because too assertive and impulsive—butt in, take things belonging to others, fail to wait their turn, & act without first considering the feelings of others	• Social problems because too passive, shy, or withdrawn.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 ADD children tend to socially isolated or withdrawn. Reading & language deficits, & 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
• There are marked similarities in the neurobiological and psychological effects of nicotine and methylphenidate. Those with ADHD are more likely to smoke than are those with ADD.
• Methylphenidate reduces catecholamine reuptake. Addressing reuptake appears to be sufficient to help most individuals with ADHD.	• A significant subset are helped by amphetamines rather than methylphenidate. Amphetamines both reduce reuptake and increase release of catecholamines. A marked deficit in the release of dopamine (DA) and norepinephrine (NE) might cause sluggishness and under-arousal.
	• People with ADD are not so much easily distracted as easily bored. Their problem lies more in motivation (under-arousal) than it does 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, though 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 affected may be a frontal-striatal one.	• The primary neural circuit affected may be a frontal-parietal one.
• Polymorphisms in the DAT1 gene are associated 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 then to ADHD. That is consistent with the centrality of prefrontal cortex in ADD because the DRD4 receptor is present in prefrontal cortex but not in the striatum in humans.

Note: DRD4, D4 dopamine receptor.