Table 3.

Sibling correlations and neuropsychological performance of the groups

					Effect Sizes for Group Comparisons			Significant Group Differences after covariance²
Measure	Sibling Correlation¹	Control M (SD)	Non-ADHD cotwins of ADHD probands M (SD)	ADHD Probands M (SD)	Control vs. ADHD	Control vs. non-ADHD cotwins	Non-ADHD cotwins vs. ADHD	IQ	Reading Ability	ADHD Symptoms
Executive Function
Response inhibition
CPT commissions	.07/.09	9.2 (15.5)	20.3 (47.6)	27.4 (44.2)	.55^***	.31^*	.15^***	C>P	C>P	C>S
SSRT	.30^*/.47^*	285.9 (109.4)	354.5 (144.5)	380.1 (159.7)	.73^***	.51^**	.22^***	C>S,P	C>S,P	C>S
Vigilance
CPT omissions	.35^*/.22^*	11.0 (11.6)	15.4 (16.0)	18.3 (15.7)	.53^***	.31^**	.18^**	None	C,S>P	C>S,P
Interference Control
Stroop	.04/.19^***	−.15 (.80)	.03 (.90)	.07 (.79)	.28^**	.21^*	.05	C>P	C>P	C>S
Set-shifting
WCST³	.20^/.27^*	13.3 (9.9)	15.7 (9.3)	20.0 (17.8)	.46^***	.25^*	.30^***	C,S>P	C,S>P	C,S>P
Trailmaking Part B	.21^*/.28^*	34.1 (19.1)	41.3 (24.8)	52.6 (33.0)	.69^***	.33^***	.39^***	C,S>P	C,S>P	C>S>P
Working Memory
Digit Span	.31^*/.32^*	11.2 (2.9)	10.1 (2.9)	9.3 (3.0)	.59^***	.32^***	.27^**	C>P	C>P	C>S,P
Counting Span	.26^*/.36^*	7.6 (2.4)	6.6 (2.7)	6.1 (2.4)	.61^***	.39^***	.19^**	C>P	C>P	C>S,P
Sentence Span	.23^*/.39^*	5.9 (2.0)	5.1 (2.3)	4.8 (2.3)	.54^***	.36^**	.16^**	C>P	None	C>S,P
CANTAB errors³	.29^*/.39^*	32.2 (16.1)	39.7 (17.6)	44.7 (18.6)	.72^***	.45^*	.28^***	C>P	C,S>P	C>P
Time Estimation⁴,⁵	.48^*/.42^	−.20 (.58)	.12 (.86)	.25 (1.01)	.54^***	.42^*	.15	None	None	None
Processing Speed
Coding	.22^*/.23^*	10.5 (2.8)	9.9 (3.0)	8.0 (3.0)	.86^***	.20^**	.63^***	C,S>P	C,S>P	C>P
Symbol Search³	.31^*/.22^*	12.9 (3.4)	12.1 (3.3)	10.7 (3.4)	.66^***	.23^*	.43^***	C,S>P	C,S>P	C>P
Rapid Naming⁵	.36^*/.20^*	.06 (1.01)	−.38 (1.0)	−.89 (1.1)	.71^***	.24^**	.47^***	C,S>P	C,S>P	C>S>P
Perceptual Speed⁵	.38^*/.29^*	−.02 (1.1)	−.36 (1.1)	−1.1 (1.1)	1.0^***	.31^***	.71^***	C,S>P	C,S>P	C>S>P
Response variability
Go RT SD	.18^*/.47^*	166.0 (45.6)	196.3 (66.6)	213.9 (76.1)	.76^***	.53^***	.25^***	C>S>P	C>S>P	C>S,P
Motivation/Delay Aversion
Doors Task⁴	.16^/.24^**	61.3 (29.1)	63.8 (29.7)	59.9 (29.5)	.05	.08	.13	None	None	None
Delay Aversion⁴	.20^/.29^**	32.7 (5.5)	32.7 (5.5)	31.7 (5.6)	.17	.00	.17	None	None	S>P

indicates significance after covarying age and sex,

p < .05;

^**

p < .01;

^***

p <.001.

Intraclass sibling correlation. Correlations in control pairs are left of the slash, and correlations between pairs with an ADHD proband are right of the slash.

C=control twins, S=siblings of ADHD probands without ADHD, P=ADHD probands, None indicates there were no group differences when covariate was included in the model. All scores are rescaled so that > indicates that the group(s) on the left of the symbol had better performance. ^{3, 4} These six tasks were added later on in the battery and have the following N’s:

Controls (n=236), Siblings (n=136), ADHD probands (n=156),

⁴

Controls (n=190), Siblings (n=92), ADHD probands (n=106).

⁵

Composite measures are residualized scores controlling age and sex.