Why do you think your paper is highly cited?

Over the past several decades, the scientific community realized that much more attention should be given to mental disorders affecting children and adolescents, given their significant and lasting negative impact. Attention-deficit/hyperactivity disorder (ADHD) is a prevalent disorder also associated with a wide range of adverse outcomes across the lifespan, justifying the scientific and lay interest in it.

This study comprehensively addresses the distribution of ADHD around the world, which is essential for estimating the burden associated with the disorder. Moreover, we further studied the sources of variability in prevalence estimates, which gives important clues on the role of geographical location of the countries included in the study.

It is important to note that the impact of cultural factors in the rates of ADHD worldwide has been a subject of intense debate. In this sense, this paper is clearly relevant for policymakers, investigators in the ADHD field, as well as those interested in research methodology, and epidemiology.

Does it describe a new discovery, methodology, or synthesis of knowledge?

The paper reports the aggregated prevalence of ADHD across 35 different countries from all world regions, including studies conducted from 1978 to 2005. Furthermore, we addressed the role of demographic location of the study in the variability of estimates using a modern statistical approach—meta-regression.

Would you summarize the significance of your paper in layman's terms?

Our study gives an estimate of the worldwide ADHD prevalence aggregating 102 studies, selected according to rigorous criteria. Moreover, since there is significant variability on estimates across studies, we investigated which factors are related to this heterogeneity.

Results indicated that different diagnostic criteria among studies, information source—who reports the symptoms (e.g., teachers or parents), and whether functional impairment is required or not for the establishment of the diagnosis, are the major factors related to the variability of estimates. However, the country where the study was conducted, which indexes at some extent the cultural background, is not related to different prevalence estimates.

How did you become involved in this research, and were there any problems along the way?

We have been studying different aspects of ADHD, from molecular genetics and pharmacogenetics to epidemiological studies. There was a clear need to summarize the extensive epidemiological literature on the disorder, but we were aware that the wide number of studies conducted in such a diverse number of countries could give us further information about the disorder.

We joined two experts in systematic reviews and meta-regression, Professors Mauricio Silva de Lima of Eli Lilly, UK and Ireland, and Bernardo Lessa Horta of the Post-Graduate Program in Epidemiology, Federal University of Pelotas, Brazil, as well as Professor Joseph Biederman of Harvard Medical School, a leader in the ADHD field, and designed the study herein highlighted.

It took us approximately two years to carefully review the literature. Two main problems occurred along the way: electronic searches showed an important degree of inaccuracy, and original studies varied greatly on the extent of information they report about their methods.

Where do you see your research leading in the future?

A wide number of studies have evidenced the significant role of genetic risk factors in the development of ADHD. Although this study indicates that cultural background has not played a main role on the worldwide variability of the occurrence of the disorder, this does not mean that environmental risk factors do not play a role in its etiological process. We should proceed with the investigation of genetic risk factors, aggregating findings from genome-wide association studies and their interaction with environmental risk factors.

Moreover, further studies must focus on the developmental trajectories of children with ADHD into adulthood, so we can better understand the disorder itself in this phase of development, and its relationship with other mental disorders, such as conduct disorder, substance use, and mood disorders.

Do you foresee any social or political implications for your research?

We can estimate the populational burden associated with ADHD more accurately with the data generated. Moreover, the conception that this disorder is more frequent in specific locations than others, due to cultural or other demographic-associated variables, is not supported by this study. In this sense, mental health authorities can use our data to estimate levels of national diagnosis of ADHD in their countries.

Guilherme V. Polanczyk, M.D., Ph.D.
Postdoctoral Research Fellow
SGDP Centre, Institute of Psychiatry
King's College London, UK and
ADHD Program
Hospital de Clinicas de Porto Alegre
Brazil

Luis Augusto Rohde, M.D., Ph.D.
Professor of Child and Adolescent Psychiatry
Federal University of Rio Grande do Sul
Brazil and
Director
ADHD Program
Hospital de Clinicas de Porto Alegre
Brazil

Keywords: attention-deficit/hyperactivity disorder, ADHD, prevalence estimates, meta-regression, demographic-associated variables, genetic risk factors, epidemiological literature.