Co-Chair, ADDA Professional Advisory Board

A great deal of research on ADHD is published every month in the professional journals in medicine, psychology, and education. It would be impossible for me to summarize all of this research. Instead, I have selected a small number of significant and interesting studies to highlight here, along with references to where you can read more about them. I will update this feature from time to time with new research findings.

More and more evidence from recent research is pointing to the fact that ADHD is passed on through heredity. Dr. Russell Barkley recently wrote a succinct, highly readable summary of research on genetics and ADHD (Barkley, 1998). For example, in this article, he mentioned how molecular genetic researchers in five different studies have discovered a link between a particular gene called the DRD4 repeater gene and ADHD. This is the same gene that has previously been associated with the personality trait of high novelty seeking. This gene is not thought to be involved in all cases of ADHD, but in a significant number. Of course, scientists are investigating many other genes too, but any consistent association of particular genes with ADHD helps us better understand what causes ADHD.

Dr. Barkley (1998) also summarized the status of twin studies. Identical twins have been found to have higher concordance rates for ADHD than fraternal twins. By concordance, we mean the chances that if one twin has ADHD, the other twin will also have it. In one study, the concordance rate was 81% for identical twins versus 29% for fraternal twins. In another study, it was 67% for identical twins and 0% for fraternal twins. These twin studies suggest that up to 80% of the variance in the trait of hyperactivity/ impulsivity is now thought to be genetically based.

This is an exciting area in which you should look for many more developments over the next few years. You may read Dr. Barkley’s article, "Gene linked to ADHD verified," in The ADHD Reports, Volume 6, Number 3, June, 1998.

Until now, there has been very little research on the effectiveness of Adderall. Dr. Swanson and his colleagues (Swanson et al., 1998) recently published a randomized, double-blind, cross-over study of the effectiveness of Adderall in 30 children with ADHD attending a specially designed laboratory classroom. The researchers compared the effectiveness of 5, 10, 15, and 20 mg. of Adderall to the best dose of Ritalin which was known to have helped each child before the child entered the study. A placebo or pill without any active ingredients was also included. The children’s behavior and academic performance was observed throughout the school day. Each child received each dose of Adderall, Ritalin, and placebo in a specially established order.

All of the doses of Adderall were superior to the placebo. Ritalin was also superior to the placebo. Adderall generally lasted longer than Ritalin, and higher doses of Adderall lasted longer than lower doses of Adderall. These effects were obtained both in terms of the children’s behavior and their academic performance. There were many more details to the study which cannot be mentioned here due to space limitations. Hopefully, there will soon be more such studies on Adderall, including some done with adults. One notable finding was that higher doses of Adderall gave a longer lasting effect. Until now, at least with Ritalin, it has commonly been thought that higher doses give a more powerful effect, but not necessarily a longer lasting effect. Apparently, this is different with Adderall than with Ritalin. Of course, this study must be replicated and extended before we can definitively accept this conclusion.

The original reference for the study is:

Swanson, J.M., Wigal, S., Greenhill, L. I., Browne, R., Waslik, B., Lerner, M., Williams, L., Flynn, D., Agler, D., Crowley, K., Finberg, E., Baren, M., and Cantwell, D.P. Analog classroom assessment of Adderall in children with ADHD. Journal of the American Academy of Child and Adolescent Psychiatry, 1998, Vol. 37, pp. 519-526.

Two recent studies have given us some new information on how ADHD impacts women at various stages of life:

Gender differences in Cognitive Abilities and Self-Ratings. Arcia and Conners (1998) examined gender differences in intellectual ability, neuropsychological performance and self-ratings of ADHD symptoms and other behavioral and emotional problems with 360 ADHD patients. The sample included 280 males and 80 females, ages 5 to 60. The Wechsler Intelligence Tests were used to measure IQ, and the Conners’ Continuous Performance Test and the Rey-Osterrieth Complex Figure Test were used to measure neuropsychological functioning. The Rey-Osterrieth Complex Figure Test is a task that requires the subject to copy a complex design and later reproduce it from memory; it taps ability to plan, organize, and assemble complex material and is a sensitive measure of the kinds of difficulties experienced by many people with ADHD. For the younger children, parents and teachers completed ratings scales of their ADHD symptoms and general functioning on the Conners’ Parent and Teacher Rating Scales. For the adolescents and the adults, they rated themselves on ADHD symptoms and general functioning on standardized rating scales.

Arcia and Conners (1998) found that there were no differences between males and females at any age on the IQ tests, the Conners’ Continuous Performance Test, or the Rey-Osterrieth Complex Figure Test. Thus, on objective measures of intellectual ability and neuropsychological functioning, males and females were similar. For the children, there were no differences between males and females on either parent or teacher rating scales. However, for the adolescents and the adults, there were significant differences between males and females on the self-ratings. Compared to the males, female adolescents and adults with ADHD rated themselves as having fewer assets, more problems of concentration, more problems of restlessness, more problems dealing with anger, less self-confidence, more emotional problems with feelings such as anxiety and depression, and more conflicts with their families.

It was indeed interesting that despite the fact that objectively speaking, males and females with ADHD looked similar on cognitive and neuropsychological measure at any age, by adolescence and adulthood, the females perceived themselves to be having more problems. They perceived themselves to be having more problems not only with ADHD symptoms such as concentration and restlessness, but also with associated factors such as anger, self-confidence, feelings, and family relations. This finding is consistent with the clinical impression of many therapists that ADHD may sometimes create a greater burden for women than for men, although there are certainly other interpretations of the data that are possible.

Readers might consult the entire article for more information:

Arcia, E., & Conners, C. K. Gender differences in ADHD? Journal of Developmental and Behavioral Pediatrics, 1998, vol. 19, pp. 77-83.

Psychological functioning of ADHD vs. control women. In another recent study, Rucklidge & Kaplan (1998) investigated the psychological functioning of 51 women diagnosed as having ADHD compared to 51 not fulfilling criteria for ADHD. The women in both groups averaged 41 years of age. The investigators used a variety of interview and self-report measures to compare the ADHD and non-ADHD women on psychiatric history, depression, anxiety, life stresses, coping styles, and self-esteem. Women with ADHD reported having had more depressive episodes in their lives, lower self-esteem, more feelings of anxiety, higher levels of stress, and more frequent involvement in psychotherapy than women without ADHD. The ADHD group also engaged in less task-oriented coping and more emotional coping than the ADHD group, and had a more external locus of control than the non-ADHD group (e.g. felt they had less control over their lives). This study clearly provides further documentation for the negative impacts of ADHD on women.

Reference: Rucklidge, J. J., & Kaplan, B. J. Psychological functioning of women identified in adulthood with Attention- Deficit/ Hyperactivity Disorder. Journal of Attention Disorders, 1997, Vol. 2, pp. 167-176.

What percentage of adults with ADHD fall into each of the major subtypes, e.g. Inattentive, Hyperactive/ Impulsive, or Combined subtypes? Which ADHD symptoms are most common in adults? Are comorbidities for other psychiatric conditions more common in one subtype of ADHD than another? These were some of the questions which Millstein, Wilens, Biederman, and Spencer (1998) recently addressed. Using a meticulous recruitment and screening/ diagnostic process involving structured psychiatric interviews, these investigators examined the functioning of 149 adults clinically referred and diagnosed as having ADHD. The sample included 88 males and 61 females, and had a mean age of 37 years. They found that 56% of the adults had the combined subtype, 37% had the inattentive subtype, and only 2% had the hyperactive/ impulsive subtype.

They analyzed which ADHD symptoms were most common in adulthood. Results indicated that the inattentive symptoms were most commonly reported, particularly difficulty sustaining attention, shifting activities frequently, and difficulty following through on tasks. Hyperactive and impulsive symptoms were less common, but nonetheless the majority of the adults reported long-standing difficulties with fidgeting, interrupting, speaking out of turn, and impatience. Hyperactive/ impulsive symptoms were reported to have decreased more from childhood to adulthood than inattentive symptoms.

Comorbidity was very common. Only 3% had no comorbidity, 11% had a lifetime history of one comorbid condition, 12% had two, 18% had three, and 56% had four or more psychiatric comorbidities. Adults with the combined subtype tended to have more comorbid psychiatric conditions than those with the inattentive or hyperactive/ impulsive subtype, particularly oppositional, bipolar, and substance use disorders. Men had higher rates of conduct disorder, antisocial disorder, alcohol and drug dependence, and stuttering than women; women had higher rates than men of depression, bulimia nervosa, and simple phobias. The authors present detailed information about the frequency of individual comorbidities, which may be of interest to readers.

This study reinforces previous research in children showing that the combined subtype of ADHD is the most common, and the hyperactive/ impulsive subtype is the least common. It also reinforces previous findings that the inattentive symptoms persist more than the hyperactive/ impulsive symptoms into adulthood, and provides valuable information about the types of comorbidities present in adults with ADHD.

Reference: Millstein, R.B., Wilens, T. E., Biederman, J., & Spencer, T.J. Presenting ADHD symptoms and subtypes in clinically referred adults with ADHD. Journal of Attention Disorders, 1998, vol. 2, 159-166.

About the author:
Arthur Robin, Ph.D., is Co-Chairperson of the ADDA Professional Advisory Board.   He is a Professor of Psychiatry and Behavioral Neurosciences, Wayne State University School Of Medicine, Detroit, MI.