■ A Taxometric Investigation of Developmental Dyslexia Subtypes Beth A. O’Brien 1, *, Maryanne Wolf 2 and Maureen W. Lovett 3 1 University of Cincinnati, Cincinnati, OH, USA 2 Tufts University, Medford, MA, USA 3 Toronto University, Toronto, Ontario, Canada Long-standing issues with the conceptualization, identification and subtyping of developmental dyslexia persist. This study takes an alternative approach to examine the heterogeneity of developmental dyslexia using taxometric classification techniques. These methods were used with a large sample of 671 children ages 6–8 who were diagnosed with severe reading disorders. Latent characteristics of the sample are assessed in regard to posited subtypes with phono- logical deficits and naming speed deficits, thus extending prior work by addressing whether these deficits embody separate classes of individuals. Findings support separate taxa of dyslexia with and without phonological deficits. Different latent structure for naming speed deficits was found depending on the definitional criterion used to define dyslexia. Non- phonologically based forms of dyslexia showed particular difficulty with naming speed and reading fluency. Copyright © 2012 John Wiley & Sons, Ltd. Practitioner Points: • Support for separate subtypes of dyslexia, with and without phonological deficits (PDs), indicates a need for different approaches to intervention. • A discrepancy-based criterion identifies more non-PD cases that may be missed with a response-to-intervention diagnosis. • Sound symbol correspondence and decoding measures may best distinguish cases of dyslexia with and without PDs. Keywords: developmental dyslexia; subtypes; double deficit hypothesis; taxometric analysis Developmental dyslexia has been the focus of a century of research across multiple fields (e.g., Orton, 1928; Geschwind & Behan, 1982; Shaywitz et al., 1992; Fletcher, 2009). Along this course, dispute regarding dyslexia’s aetiology, definition, identification and even its ex- istence has persisted. But one historical trend in any conceptualization of dyslexia is the recurrent view that the ‘syndrome’ is composed of separate syndromes, usually referred to as subtypes. Within this perspective, delineating subtypes is key to establishing both the aetiology and diagnosis of developmental dyslexia. Research efforts in the past few decades have led to great gains in reconceptualizing developmental dyslexia as a neurodevelopmental syndrome (Pennington, 2009), and this process has contributed to improved approaches to remediation. The most recently conceived defining characteristics are more inclusionary than in the past, including specific difficulties in word reading linked to phonological processing (Lyon, Shaywitz, & Shaywitz, 2003). Although it is widely held that dyslexia is a heterogeneous disorder, the prevailing *Correspondence to: Beth A. O’Brien, School of Education, CECH, University of Cincinnati, 2150 Edwards One, PO Box 210105, Cincinnati, OH 45221-0105, USA. E-mail: obrienba@uc.edu Copyright © 2012 John Wiley & Sons, Ltd. DYSLEXIA 18: 16–39 (2012) DYSLEXIA Published online 8 January 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/dys.1431