Table S3.

Mean scores and results of between-family analyses (NCDS participants)

	Sibship size	n	Birth order				Effect of sibship size		Effect of birth order
			1	2	3	4	F	P	F	P
Extraversion	2	2,018	50.09	49.85					0.29	0.588
	3	1,571	50.23	50.02	50.70				0.58	0.559
	4	900	51.22	49.40	49.19	48.84			2.46	0.061
		4,489					0.46	0.633	1.10	0.346
Emotional stability	2	2,018	50.59	50.11					1.20	0.274
	3	1,571	50.02	50.26	49.73				0.33	0.718
	4	900	49.95	50.22	49.62	48.80			0.72	0.540
		4,489					0.25	0.776	0.93	0.427
“I get upset easily“, “I get irritated easily“	2	2,010	49.52	50.40					3.57	0.059
	3	1,558	50.13	49.85	50.28				0.21	0.810
	4	890	49.77	49.42	49.79	51.52			1.59	0.190
		4,458					0.44	0.643	1.97	0.117
“I often feel blue“, “I seldom feel blue“	2	2,010	49.63	49.94					0.49	0.485
	3	1,558	49.54	50.03	50.51				1.17	0.312
	4	890	50.08	50.76	50.59	50.59			0.20	0.898
		4,458					0.71	0.492	1.01	0.389
Agreeableness	2	2,018	50.20	50.03					0.15	0.703
	3	1,571	49.50	49.63	50.91				3.03	0.049
	4	900	49.79	50.14	50.12	49.76			0.10	0.961
		4,489					0.84	0.433	1.51	0.209
Conscientiousness	2	2,018	50.67	50.15					1.39	0.239
	3	1,571	50.23	50.01	50.53				0.36	0.700
	4	900	50.38	49.58	48.67	48.98			1.24	0.294
		4,489					1.64	0.194	0.83	0.475
Openness to experience	2	2,018	50.82	50.52					0.47	0.493
	3	1,571	51.08	50.39	50.22				1.14	0.321
	4	900	50.19	49.96	48.79	47.49			3.15	0.024
		4,489					2.10	0.123	3.57	0.014
Imagination	2	2,018	50.24	50.32					0.04	0.849
	3	1,571	50.82	50.34	50.26				0.52	0.596
	4	900	50.36	49.54	49.25	48.78			0.98	0.401
		4,489					1.54	0.214	0.84	0.470
Intellect	2	2,018	51.21	50.63					1.72	0.190
	3	1,571	51.10	50.38	50.20				1.28	0.279
	4	900	50.07	50.41	48.75	47.03			4.80	0.003
		4,489					2.00	0.136	5.32	0.001
Intelligence (IQ)	2	3,168	103.70	102.71					3.87	0.050
	3	2,526	102.39	100.55	99.68				7.60	0.001
	4	1,562	99.31	99.23	98.48	95.40			4.65	0.003
		7,256					28.03	<0.001	10.40	<0.001