title: Understanding the complex etiologies of developmental disorders: behavioral and molecular genetic approaches.
abstract: OBJECTIVE
  This article has 2 primary goals. First, a brief tutorial on behavioral and molecular genetic methods is provided for readers without extensive training in these areas. To illustrate the application of these approaches to developmental disorders, etiologically informative studies of reading disability (RD), math disability (MD), and attention-deficit hyperactivity disorder (ADHD) are then reviewed. Implications of the results for these specific disorders and for developmental disabilities as a whole are discussed, and novel directions for future research are highlighted.
  
  
  METHODS
  Previous family and twin studies of RD, MD, and ADHD are reviewed systematically, and the extensive molecular genetic literatures on each disorder are summarized. To illustrate 4 novel extensions of these etiologically informative approaches, new data are presented from the Colorado Learning Disabilities Research Center, an ongoing twin study of the etiology of RD, ADHD, MD, and related disorders.
  
  
  CONCLUSIONS
  RD, MD, and ADHD are familial and heritable, and co-occur more frequently than expected by chance. Molecular genetic studies suggest that all 3 disorders have complex etiologies, with multiple genetic and environmental risk factors each contributing to overall risk for each disorder. Neuropsychological analyses indicate that the 3 disorders are each associated with multiple neuropsychological weaknesses, and initial evidence suggests that comorbidity between the 3 disorders is due to common genetic risk factors that lead to slow processing speed.
authors.affiliation: Department of Psychology and Neuroscience, University of Colorado at Boulder, Boulder, CO 80309, USA. willcutt@colorado.edu
authors.forename: Erik G
authors.initials: EG
authors.lastname: Willcutt
authors.forename: Bruce F
authors.initials: BF
authors.lastname: Pennington
authors.forename: Laramie
authors.initials: L
authors.lastname: Duncan
authors.forename: Shelley D
authors.initials: SD
authors.lastname: Smith
authors.forename: Janice M
authors.initials: JM
authors.lastname: Keenan
authors.forename: Sally
authors.initials: S
authors.lastname: Wadsworth
authors.forename: John C
authors.initials: JC
authors.lastname: Defries
authors.forename: Richard K
authors.initials: RK
authors.lastname: Olson
mesh_terms: D001289:Attention Deficit Disorder with Hyperactivity / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology*; D002648:Child; D002652:Child Behavior / Q000523:psychology*; D015897:Comorbidity; D002658:Developmental Disabilities / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology*; D004410:Dyslexia / Q000235:genetics / Q000523:psychology; D005190:Family; D020022:Genetic Predisposition to Disease; D006801:Humans; D007859:Learning Disabilities / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology*; D008433:Mathematics; D012307:Risk Factors; D012931:Social Environment
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