Table 1.

Prevalence and demographic correlates of intellectual disability compared with those with no intellectual disability, and its components (intelligence and adaptive behavior) in a population-representation sample of adolescents

		Dichotomous ID status			IQ and AB components
	Total	No ID % (ref)	ID %	χ2; p value	No low IQ, no low AB % (ref)	ID %	χ2; p value	Low IQ, no Low AB %	χ2; p value	No low IQ, low AB %	χ2; p value
Total	100	96.8	3.2		74.1	3.2		3.3		19.6
Gender
Girls	51.5	48.9	44.9	0.3	51.8	44.9	0.335	35.6	7.2	39.5	15.6
Boys	48.5	51.1	55.1	0.571	48.2	55.1		64.4	0.007	60.5	<0.0001
Age (years)
13	17.2	16.7	13.5	0.517	16.8	13.5	0.511	11.7	0.633	17.5	0.908
14	23.1	21.6	27.7		21.5	27.7		24.8		21.5
15–16	39.1	42.4	37.0		42.6	37.0		45.2		40.9
17–18	20.6	19.3	21.8		19.2	21.8		18.3		20.1
Income to poverty line ratio
<1.5	14.2	11.9	24.5	15.2	10.3	24.5	20.2	14.3	0.700	17.8	27.1
1.5–2.9	18.8	17.8	21.5	0.002	17.2	21.5	0.000	17.6		20.0	<0.0001
3–5.9	33.0	34.3	23.0		35.0	23.0		35.4		31.2
≥6	34.0	36.1	31.0		37.5	31.0		32.7		30.9
Parent education
Less than high school	11.3	9.8	25.2	20.2	8.5	25.2	24.8	7.2	0.437	15.6	45.3
High school graduate	28.6	28.0	38.3	0.0002	27.0	38.3	<0.0001	35.3		30.9	<0.0001
Some college	21.0	21.1	17.2		20.9	17.2		16.8		22.8
College degree	39.2	41.0	19.4		43.6	19.4		40.8		30.7
Number of biological parents at home
None	8.1	7.6	19.7	25.2	6.3	19.7	34.5	4.8	0.718	13.5	68.3
One	35.0	33.7	42.9	<0.0001	31.3	42.9	<0.0001	31.4		44.1	<0.0001
Two	56.9	58.7	37.4		62.4	37.4		63.8		42.4
Parent substance misuse
No	95.4	94.9	90.8	0.128	95.3	90.8	0.071	97.3	0.268	92.7	0.068
Yes	4.6	5.1	9.2		4.7	9.2		2.7		7.3
Parent psychopathology
No	92.5	92.0	87.9	0.179	92.7	87.9	0.117	91.1	0.581	89.5	5.4
Yes	7.5	8.0	12.1		7.3	12.1		8.9		10.5	0.020
Any traumatic event exposure
No	41.9	42.5	43.2	0.918	44.5	43.2	0.855	43.7	0.875	34.1	15.5
Yes	58.1	57.5	56.8		55.5	56.8		56.3		65.9	<0.0001

ID = Intellectual disability, defined as IQg ≤76 & ABg or any factor score ≤76.

IQ = Intelligence Quotient; AB = Adaptive behavior.