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Developmental Neuropsychology Lab

Current Projects

Validity of Subtypes of ADHD

A Component of the Colorado Learning Disability Research Center (CLDRC) funded by the National Institute of Child Health and Human Development

Principal Investigators:
Bruce F. Pennington, Ph.D. and Erik Willcutt, Ph.D.


This twin study extends our previous work on the genetics and neuropsychology of ADHD (see selected annotated publications), by testing the validity of subtypes of ADHD using converging methods. Despite recent progress in identifying neurobiological mechanisms in ADHD, there is considerable heterogeneity both within and across the clinical phenotypes of ADHD defined by the DSM-IV. This heterogeneity is evident in situational and reporter differences, patterns of comorbidities, neuropsychological correlates, medication response, and possibly genetic mechanisms. Some of this heterogeneity is likely due to genuine biological differences, whereas some of it may reflect errors in measurement. This continuing twin study will test the validity of:

  1. DSM-IV subtypes,
  2. Those defined by comorbidities,
  3. Those defined by situational and reporter differences, and
  4. Those defined by age and gender.

Internal validity will be tested by factor analyses and by whether subtypes are familial within twin pairs. External validity will be tested using both behavioral and molecular genetic methods, neuropsychological measures, and measures of clinical correlates and functional impairments.

To accomplish these goals, we will administer a battery of neuropsychological and psychiatric measures to the samples of RD, ADHD, and control twin pairs (100 pairs in each group, half MZ and half DZ) and their siblings who will also be tested in other projects of the CLDRC.

The neuropsychological battery of our study is designed to test further the executive deficit hypothesis of ADHD by pitting it against a motivational deficit hypothesis. While a deficit on measures of inhibition, such as the Stop task, is a well-replicated correlate of ADHD, the relation between inhibitory and other executive measures and ADHD is far from perfect. Thus, an executive deficit may only characterize a subtype of ADHD, or it may only be a correlate of a more fundamental deficit, or the executive deficit may interact with a second deficit. To test these possibilities, we have added measures to test the motivational deficit hypothesis, in addition to continuing to collect data on executive measures correlated with ADHD. A particular focus of this study is the comorbidity between ADHD and dyslexia. We are testing which genetic and cognitive risk factors are shared and not shared by these two disorders.