Table S6.

Intellect scores before and after adjusting for intelligence (IQ) and results of corresponding between-family analyses (NCDS and SOEP participants)

	Sibship size	n	Mean scores by birth-order position				Effect of sibship size		Effect of birth order
			1	2	3	4	F	P	F	P
Intellect	2	3,562	51.47	50.26					13.73	<0.001
	3	2,435	51.38	50.41	49.89				4.81	0.001
	4	1,299	50.78	50.46	49.80	47.49			6.62	<0.001
		7,296					0.15	0.858	13.55	<0.001
Intellect, adjusted for IQ scores	2	3,562	51.39	50.35					10.80	0.001
	3	2,435	51.18	50.62	49.90				3.72	0.024
	4	1,299	50.49	50.38	50.01	47.78			5.52	0.001
		7,296					1.75	0.174	10.13	<0.001

In the NLSY, we were not able to decompose openness to experience into imagination and intellect due to the content of the questionnaire. Furthermore, we were not able to investigate whether the effect on intellect persisted after adjustment for IQ in a within-family design, because (i) in the NCDS there was no within-family data included due to the design of the study; and (ii) in the within-family sample of the SOEP were not enough individuals with information on IQ (n = 141).