Table 1.

Sample Characteristics

	Clinic-Referred Sample	Twin Sample	Total
Sample Size
# of families	138	83	221
Total children	224	156	380
# of probands	135	NA	135
# of siblings	89	53	142
# of controls	NA	103	103
Diagnostic Information
Probands
With ADHD	104 (I-43, H-5, C-56)	NA	104 (I-43, H-5, C-56)
With ODD	41	NA	41
With CD	3	NA	3
Siblings
With ADHD	16 (I-9, H-5, C-2)	34 (I-20, H-5, C-9)	50 (I-29, H-10, C-11)
With ODD	30	0	30
With CD	4	0	4
Demographic Characteristics
Age*a	11.2 (3.3)	13.5 (2.4)	12.2 (3.2)
Gender*b	144 M (64%), 80 F	61 M (39%), 95 F	205 M, 175 F
Ethnicity
Hispanic	2 (1%)	0 (0%)	2
European-American	170 (76%)	134 (86%)	304
African-American*c	18 (8%)	4 (3%)	22
Other*d	23 (10%)	2 (1%)	25
Missing	11 (5%)	16 (10%)	27

Note. ADHD = Attention-Deficit/Hyperactivity Disorder; ADHD diagnostic subtype information – I = Predominantly Inattentive type, H = Predominantly Hyperactive-Impulsive type, C = Combined type; ODD = Oppositional Defiant Disorder; CD = Conduct Disorder; Gender information – M = Male, F = Female;

^*Clinic-referred and twin samples significantly differed;

^at(378) = 7.80, p < .001;

^bχ²(1, N = 380) = 23.47, p < .001;

^cχ²(1, N = 380) = 5.03, p = .025;

^dχ²(1, N = 380) = 8.73, p = .003