Review article on 2e from NAGC researchers, which estimates prevalence as ~300,000 in the USA:
http://maxwellgate.pbworks.com/w/fi...calInvestigationTwice-Exceptionality.pdfSame information, in an NAGC white paper:
https://www.nagc.org/sites/default/files/Position%20Statement/twice%20exceptional.pdfI've also seen 2-5%. The research is not particularly clear, as investigators use different definitions of both categories of exceptionality.
More recent research, looking at the % of disabled children who were assessed at 120+ (90+ %ile) on at least one WJIII achievement measure. They came up with 9% of disabled students:
http://masseyuniversity.mrooms.net/...a%20Special%20Education%20Population.pdfThere is some evidence that ADHD is evenly distributed across the cognition bell curve, and that LD is not (the previous citation references 36% of LD students potentially identifiable as gifted--keeping in mind that most standard definitions of LD rule out everyone below average a priori, so you're starting the discussion from only the upper half of the population).
In HG+ kids, even less is known.
This is an interesting local effort to identify 2e learners, which focused on already identified GT students (90+%ile), and found about 15-20% of them were 2e. It also includes what they did to try to remediate the other exceptionality.
http://www.stthomas.edu/media/project2excel/pdf/RevisedCombinedP2XAdminstratorUpdate2011.pdf
