Table 1.

Tests of statistical significance for effects of birth-order on intelligence and personality

Trait	Between-family analyses								Within-family analyses
	Combined sample		NCDS		NLSY*		SOEP		Combined sample		NLSY*		SOEP†
	F	P	F	P	F	P	F	P	F	P	F	P	F	P
Intelligence (IQ)	11.80	<0.001	10.40	<0.001	3.87	0.009	2.69	0.045			11.82	<0.001
Extraversion	0.62	0.600	1.10	0.346	1.50	0.212	1.53	0.204	1.87	0.132	1.88	0.131	0.22	0.883
Emotional stability	0.57	0.638	0.93	0.427	0.43	0.729	0.65	0.584	1.17	0.319	0.12	0.946	2.24	0.083
Agreeableness	0.26	0.858	1.51	0.209	0.77	0.512	2.11	0.096	0.76	0.517	0.90	0.442	0.42	0.740
Conscientiousness	0.25	0.863	0.83	0.475	0.48	0.695	1.23	0.296	0.17	0.914	0.13	0.944	0.15	0.928
Openness	3.64	0.012	3.57	0.014	0.06	0.979	1.97	0.116	1.70	0.164	1.29	0.277	0.43	0.733
Imagination	1.43	0.232	0.84	0.470			0.76	0.516					0.55	0.647
Intellect	13.32	<0.001	5.32	0.001			9.33	<0.001					4.59	0.004

The sample sizes used in these analyses varied between 2,914 and 17,030 for the between-family analyses and between 1,094 and 3,156 for the within-family analyses. See Table S1 for the specific sample sizes.

^*In the NLSY, due to the content of the questionnaire, openness to experience could not be decomposed into imagination and intellect.

^†In the within-family sample of the SOEP, there were not enough individuals with information on IQ (n = 141) to conduct meaningful analyses, given the small effect sizes that were expected.