Table 1.
Concordance rates for RD and ADHD in pairs of MZ and DZ twins
| Number of pairs |
Probandwise Concordance |
|||
|---|---|---|---|---|
| MZ | DZ | MZ | DZ | |
| Reading Disorder | ||||
| Bakwin (1973)126 | 31 | 31 | 91% | 45% |
| Harlaar et al. (2005)32 | 308 | 246 | 72% | 45% |
| Hawke et al. (2006)127 | 306 | 247 | 66% | 35% |
| Stevenson et al. (1987)24 | 14 – 18a | 27 – 38a | 33 – 50%a | 29 – 54%a |
| Zerbin-Rudin (1967)128 | 17 | 34 | 100% | 52% |
| Weighted average | 70% | 41% | ||
| ADHD | ||||
| Goodman & Stevenson (1989)129 | 39 | 54 | 51% | 33% |
| Levy et al. (1997)49 | 57 | 46 | 82% | 38% |
| Levy et al. (2001)130 | 138 | 109 | 67% | 42% |
| Sherman et al. (1997)131 | 69 | 32 | 58% | 31% |
| Thapar et al. (2001)132 | 175b | 410b | 79% | 54% |
| Todd et al. (2001)133 | 72 | 135 | 68% | 22% |
| Willcutt et al. (2000)25 | 88 | 82 | 78% | 38% |
| Willcutt et al. (2007)38 | 83 | 78 | 68% | 24% |
| Weighted average | 71% | 41% | ||
| Math Disorder | ||||
| Alarcón et al. (1997)134 | 63 | 32 | 76% | 56% |
| Kovas et al. (2007)16 | 93 | 83 | 40% | 24% |
| Weighted average | 55% | 33% | ||
a
concordance rates were provided for several different definitions of RD. The average MZ and DZ concordance was use for the weighted average.
b
specific Ns were not provided, so Ns are estimated based on the total sample and the 80th percentile threshold used to define the extreme ADHD probands.