- Email: [email protected]
In Defense of Lumping (and Splitting)
LETTERS TO THE EDITOR
In Defense of Lumping (and Splitting) To the Editor: n any taxonomic endeavor there will be debates between “lumpers” and “splitters.” Lumpers emphasize the similarities between phenomena and classify them in large inclusive taxa. Splitters, by contrast, focus on differences, resulting in complex classification systems with many categories. In his editorial on our recent studies, which critically evaluated how autism spectrum disorders (ASDs) are to be defined in the DSM-5,1,2 Dr. Leventhal3 described us (correctly) as lumpers and elegantly outlined some arguments in favor of splitting. His point was not that splitting is better than lumping, but rather, that each approach carries risks and advantages. Using the example of ASD, we write to make the case for lumping in the DSM-5 and to suggest a way in which the inherent risks of this approach can be mitigated. We start from the following axiom: “classifications are cognitive structures imposed on data to achieve particular goals.”4 What is the primary goal of the DSM-5? It is to facilitate good care of patients—to promote clinical utility. As such, DSM-5 diagnoses should be evaluated according to their reliability and how much information they contain about associated difficulties, prognosis, and treatment needs. Previous classifications, which entailed splitting the autism spectrum, had limited reliability and did not identify groups that were distinct in terms of their prognosis, treatment needs, or comorbidity. In the interests of clinical utility, given our current state of knowledge, the DSM-5 is correct to take a lumping approach to ASD symptoms. However, Dr. Leventhal’s3 arguments for splitting were based less on concerns of clinical utility and more on the need for nosology to serve scientific ends. He argues convincingly that the DSM-5 ASD is likely to “lump important factors that may have distinct biological substrates.” Were we to base our research into the etiology of ASDs on an assumption that there is a common pathway to the spectrum of disorders adumbrated by the DSM-5, we would be indeed at risk of hindering the discovery of the diverse gene-brain-behavior pathways that underpin autistic symptoms.
I
How can we maximize the clinical utility of an ASD category without excessively compromising its validity in a way that impedes scientific progress? One solution would be to have separate nosologies for clinicians and researchers. In our view, such a “Tower of Babel” approach is unwise, because it would undermine links between research and clinical practice. Instead, we suggest that the potential etiologic heterogeneity of the autism spectrum, in terms of dysfunctional neural mechanisms with their biological substrates, is not best captured at the descriptive level of symptomatology. Rather, we should encourage the study of endophenotypes. Within the DSM-5 ASD category, there may exist valid subgroups that can be defined (split off), not by overt symptoms, but by covert characteristics. Evidence is already emerging that within the autism spectrum there is substantial variability in some cognitive skills, such as the capacity for social recognition or executive abilities.5 As long as those seeking to understand the etiology of ASD bear in mind the presence of complexity and diversity below the level of overt symptoms, there is no reason for their work to be impeded by the lumping tendencies of the DSM-5 task force. The American Psychiatric Association is committed to making the DSM-5 a living document. We hope and believe that endophenotypic research in ASD will ultimately fulfill its promise, resulting in the identification of valid, etiologically distinct groups on the autism spectrum. If it is demonstrated that it makes clinical sense to distinguish such subgroups, the DSM should be updated accordingly. However, the idea that the autism spectrum should undergo a nosologic split is nothing more or less than an aspiration, requiring empirical testing.
JOURNAL OF THE AMERICAN ACADEMY OF CHILD & ADOLESCENT PSYCHIATRY VOLUME 51 NUMBER 4 APRIL 2012
William P.L. Mandy, D.Clin.Psy. David H. Skuse, M.D. University College London London, UK [email protected]
Tony Charman,
Ph.D.
Centre for Research in Autism and Education Institute of Education London, UK
www.jaacap.org
441
LETTERS TO THE EDITOR
Thomas W. Frazier,
Ph.D.
Center for Pediatric Behavioral Health and Center for Autism Cleveland Clinic Cleveland, OH
Disclosure: Dr. Skuse is a stockholder in IxDx, Ltd., which owns exclusive rights to the interview software and to the dissemination of 3Di technology and intellectual property. Dr. Charman receives grant or research support from the Medical Research Council, the European Science Foundation, Autistica, the Autism Education Trust, and the Department of Education. He receives royalties from Guilford Press and Sage. Dr. Frazier has received federal funding or research support from, acted as a consultant to, or received travel support from Integragen, Shire Development, Bristol-Myers Squibb, the National Center for Research Resources, and the Brain and Behavior Research Foundation. Dr. Mandy reports no biomedical financial interests or potential conflicts of interest. 0890-8567/$36.00/©2012 American Academy of Child and Adolescent Psychiatry DOI: 10.1016/j.jaac.2012.02.004
REFERENCES 1. Frazier TW, Youngstrom EA, Speer L et al. Validation of proposed DSM-5 criteria for autism spectrum disorder. J Am Acad Child Adolesc Psychiatry. 2012;51:28-40. 2. Mandy WPL, Charman T, Skuse DH. Testing the construct validity of proposed criteria for DSM-5 autism spectrum disorder. J Am Acad Child Adolesc Psychiatry. 2012;51:41-50. 3. Leventhal BL. Lumpers and splitters: who knows? Who cares? J Am Acad Child Adolesc Psychiatry. 2012;51:6-7. 4. Hyman SE. The diagnosis of mental disorders: the problem of reification. Annu Rev Clin Psychol. 2010;6:155-179. 5. Charman T, Jones CRG, Pickles A, Simonoff E, Baird G, Happé F. Defining the cognitive phenotype of autism. Brain Res. 2011;1380:10-21.
Dr. Leventhal replies: am grateful for the comments on my editorial, “Lumpers, Splitters: Who Knows? Who Cares?”1 I am glad that my colleagues understood that I was not offering support for either lumping or splitting, rather, I am calling attention to the fact that each has advantages and disadvantages about which we should be aware so we do not expect or promise more than the DSM-5, or any other classification system allows. In their comment on my editorial, my colleagues raise some excellent points. I would like to respond to four of their assertions. 1. The primary goal of DSM-V “. . . is to facilitate good patient care.” Surely, the ultimate goal of all clinical practice and all research is to facilitate good patient care. However, practitioners and investigators (who are often also practitioners) may need to use diagnostic criteria for very different, albeit related, purposes. Indeed, the American Psychiat-
I
ric Association asserts that the DSM’s are intended to be play a role in both clinical practice and research.2 Further, one only needs to think back but a few years to recall that the DSM’s were really evolved from the research of Robins and Guze and the “Feighner research diagnostic criteria.”3 Of course, this is part of the conundrum: the DSM’s have many masters and many uses; the DSM committees must make considered compromises to allow for these varied uses, ensuring it is good enough for each and recognizing that it may not be perfect for any. 2. If we lump important factors that may have distinct biological substrates, we assume a common causal pathway for “. . . the spectrum of disorders adumbrated by the DSM-V . . .” To be honest, I did not know the word adumbrated (for those of you who share my ignorance, according to the Oxford English Dictionary, it is the past participle of adumbrate, a verb that means “represented faintly or in outline”). Well, the DSM’s are “outlines” that leave some room for clinicians and scientists to fill in the spaces as knowledge and experience expands. This is a great strength of the DSM’s and one of their weaknesses; but, in no way, should a single set of diagnostic criteria lead to the conclusion that there is a common etiologic pathway. Indeed, I addressed this in my editorial comments about genetic heterogeneity versus pleiotropism, each of which is affected by the decisions about whether to lump or split. 3. The etiologic heterogeneity of the autism spectrum “. . . is not best captured in the descriptive level of symptomatology. Rather, we should encourage the study of endophenotypes.” I agree that the presumed etiologic heterogeneity of Autism Spectrum Disorder (ASD) may not best be captured by descriptive symptomatology (from which one can also construct important endophenotypes). But, for the moment, it is the best that we have and, given the limits of this method, we are not doing so badly in developing treatments (good patient care!) for ASD and identifying specific etiologic elements for at least a small percentage of individuals with ASD. The DSM-IV has been helpful in leading us to a good start in this process and I suspect that the DSM-5, with whatever flaws it may have, will help clinicians and investigators get even further down this long road. 4. There is a way in which the inherent risks of lumping “. . . can be mitigated.” If there was any element of my editorial that I wanted the reader to take away, it was the notion
JOURNAL 442
www.jaacap.org
OF THE
AMERICAN ACADEMY OF CHILD & ADOLESCENT PSYCHIATRY VOLUME 51 NUMBER 4 APRIL 2012
In Defense of Lumping (and Splitting) To the Editor: n any taxonomic endeavor there will be debates between “lumpers” and “splitters.” Lumpers emphasize the similarities between phenomena and classify them in large inclusive taxa. Splitters, by contrast, focus on differences, resulting in complex classification systems with many categories. In his editorial on our recent studies, which critically evaluated how autism spectrum disorders (ASDs) are to be defined in the DSM-5,1,2 Dr. Leventhal3 described us (correctly) as lumpers and elegantly outlined some arguments in favor of splitting. His point was not that splitting is better than lumping, but rather, that each approach carries risks and advantages. Using the example of ASD, we write to make the case for lumping in the DSM-5 and to suggest a way in which the inherent risks of this approach can be mitigated. We start from the following axiom: “classifications are cognitive structures imposed on data to achieve particular goals.”4 What is the primary goal of the DSM-5? It is to facilitate good care of patients—to promote clinical utility. As such, DSM-5 diagnoses should be evaluated according to their reliability and how much information they contain about associated difficulties, prognosis, and treatment needs. Previous classifications, which entailed splitting the autism spectrum, had limited reliability and did not identify groups that were distinct in terms of their prognosis, treatment needs, or comorbidity. In the interests of clinical utility, given our current state of knowledge, the DSM-5 is correct to take a lumping approach to ASD symptoms. However, Dr. Leventhal’s3 arguments for splitting were based less on concerns of clinical utility and more on the need for nosology to serve scientific ends. He argues convincingly that the DSM-5 ASD is likely to “lump important factors that may have distinct biological substrates.” Were we to base our research into the etiology of ASDs on an assumption that there is a common pathway to the spectrum of disorders adumbrated by the DSM-5, we would be indeed at risk of hindering the discovery of the diverse gene-brain-behavior pathways that underpin autistic symptoms.
I
How can we maximize the clinical utility of an ASD category without excessively compromising its validity in a way that impedes scientific progress? One solution would be to have separate nosologies for clinicians and researchers. In our view, such a “Tower of Babel” approach is unwise, because it would undermine links between research and clinical practice. Instead, we suggest that the potential etiologic heterogeneity of the autism spectrum, in terms of dysfunctional neural mechanisms with their biological substrates, is not best captured at the descriptive level of symptomatology. Rather, we should encourage the study of endophenotypes. Within the DSM-5 ASD category, there may exist valid subgroups that can be defined (split off), not by overt symptoms, but by covert characteristics. Evidence is already emerging that within the autism spectrum there is substantial variability in some cognitive skills, such as the capacity for social recognition or executive abilities.5 As long as those seeking to understand the etiology of ASD bear in mind the presence of complexity and diversity below the level of overt symptoms, there is no reason for their work to be impeded by the lumping tendencies of the DSM-5 task force. The American Psychiatric Association is committed to making the DSM-5 a living document. We hope and believe that endophenotypic research in ASD will ultimately fulfill its promise, resulting in the identification of valid, etiologically distinct groups on the autism spectrum. If it is demonstrated that it makes clinical sense to distinguish such subgroups, the DSM should be updated accordingly. However, the idea that the autism spectrum should undergo a nosologic split is nothing more or less than an aspiration, requiring empirical testing.
JOURNAL OF THE AMERICAN ACADEMY OF CHILD & ADOLESCENT PSYCHIATRY VOLUME 51 NUMBER 4 APRIL 2012
William P.L. Mandy, D.Clin.Psy. David H. Skuse, M.D. University College London London, UK [email protected]
Tony Charman,
Ph.D.
Centre for Research in Autism and Education Institute of Education London, UK
www.jaacap.org
441
LETTERS TO THE EDITOR
Thomas W. Frazier,
Ph.D.
Center for Pediatric Behavioral Health and Center for Autism Cleveland Clinic Cleveland, OH
Disclosure: Dr. Skuse is a stockholder in IxDx, Ltd., which owns exclusive rights to the interview software and to the dissemination of 3Di technology and intellectual property. Dr. Charman receives grant or research support from the Medical Research Council, the European Science Foundation, Autistica, the Autism Education Trust, and the Department of Education. He receives royalties from Guilford Press and Sage. Dr. Frazier has received federal funding or research support from, acted as a consultant to, or received travel support from Integragen, Shire Development, Bristol-Myers Squibb, the National Center for Research Resources, and the Brain and Behavior Research Foundation. Dr. Mandy reports no biomedical financial interests or potential conflicts of interest. 0890-8567/$36.00/©2012 American Academy of Child and Adolescent Psychiatry DOI: 10.1016/j.jaac.2012.02.004
REFERENCES 1. Frazier TW, Youngstrom EA, Speer L et al. Validation of proposed DSM-5 criteria for autism spectrum disorder. J Am Acad Child Adolesc Psychiatry. 2012;51:28-40. 2. Mandy WPL, Charman T, Skuse DH. Testing the construct validity of proposed criteria for DSM-5 autism spectrum disorder. J Am Acad Child Adolesc Psychiatry. 2012;51:41-50. 3. Leventhal BL. Lumpers and splitters: who knows? Who cares? J Am Acad Child Adolesc Psychiatry. 2012;51:6-7. 4. Hyman SE. The diagnosis of mental disorders: the problem of reification. Annu Rev Clin Psychol. 2010;6:155-179. 5. Charman T, Jones CRG, Pickles A, Simonoff E, Baird G, Happé F. Defining the cognitive phenotype of autism. Brain Res. 2011;1380:10-21.
Dr. Leventhal replies: am grateful for the comments on my editorial, “Lumpers, Splitters: Who Knows? Who Cares?”1 I am glad that my colleagues understood that I was not offering support for either lumping or splitting, rather, I am calling attention to the fact that each has advantages and disadvantages about which we should be aware so we do not expect or promise more than the DSM-5, or any other classification system allows. In their comment on my editorial, my colleagues raise some excellent points. I would like to respond to four of their assertions. 1. The primary goal of DSM-V “. . . is to facilitate good patient care.” Surely, the ultimate goal of all clinical practice and all research is to facilitate good patient care. However, practitioners and investigators (who are often also practitioners) may need to use diagnostic criteria for very different, albeit related, purposes. Indeed, the American Psychiat-
I
ric Association asserts that the DSM’s are intended to be play a role in both clinical practice and research.2 Further, one only needs to think back but a few years to recall that the DSM’s were really evolved from the research of Robins and Guze and the “Feighner research diagnostic criteria.”3 Of course, this is part of the conundrum: the DSM’s have many masters and many uses; the DSM committees must make considered compromises to allow for these varied uses, ensuring it is good enough for each and recognizing that it may not be perfect for any. 2. If we lump important factors that may have distinct biological substrates, we assume a common causal pathway for “. . . the spectrum of disorders adumbrated by the DSM-V . . .” To be honest, I did not know the word adumbrated (for those of you who share my ignorance, according to the Oxford English Dictionary, it is the past participle of adumbrate, a verb that means “represented faintly or in outline”). Well, the DSM’s are “outlines” that leave some room for clinicians and scientists to fill in the spaces as knowledge and experience expands. This is a great strength of the DSM’s and one of their weaknesses; but, in no way, should a single set of diagnostic criteria lead to the conclusion that there is a common etiologic pathway. Indeed, I addressed this in my editorial comments about genetic heterogeneity versus pleiotropism, each of which is affected by the decisions about whether to lump or split. 3. The etiologic heterogeneity of the autism spectrum “. . . is not best captured in the descriptive level of symptomatology. Rather, we should encourage the study of endophenotypes.” I agree that the presumed etiologic heterogeneity of Autism Spectrum Disorder (ASD) may not best be captured by descriptive symptomatology (from which one can also construct important endophenotypes). But, for the moment, it is the best that we have and, given the limits of this method, we are not doing so badly in developing treatments (good patient care!) for ASD and identifying specific etiologic elements for at least a small percentage of individuals with ASD. The DSM-IV has been helpful in leading us to a good start in this process and I suspect that the DSM-5, with whatever flaws it may have, will help clinicians and investigators get even further down this long road. 4. There is a way in which the inherent risks of lumping “. . . can be mitigated.” If there was any element of my editorial that I wanted the reader to take away, it was the notion
JOURNAL 442
www.jaacap.org
OF THE
AMERICAN ACADEMY OF CHILD & ADOLESCENT PSYCHIATRY VOLUME 51 NUMBER 4 APRIL 2012