Table S9.

Mean scores from sibships with two children of the same sex and results of the corresponding between and within-family analyses (NLSY and SOEP participants)

	Sex	Between-family					Within-family
		n	Observed mean scores by birth-order position		Effect of birth order		n	Estimated mean scores by birth-order position		Effect of birth order
			Firstborn	Second-born	F	P		Firstborn	Second-born	F	P
Extraversion	Male	1,625	48.86	49.45	1.52	0.218	339	48.49	48.67	0.06	0.805
	Female	1,755	50.83	51.50	1.91	0.167	318	50.83	51.15	0.09	0.760
Emotional stability	Male	1,625	52.50	51.96	1.51	0.220	339	52.60	52.21	0.15	0.698
	Female	1,755	47.93	48.63	2.09	0.148	318	49.21	47.81	2.22	0.139
Agreeableness	Male	1,625	47.86	48.11	0.21	0.646	339	47.49	49.14	2.95	0.087
	Female	1,755	51.90	51.90	0.00	0.997	318	51.97	51.70	0.09	0.764
Conscientiousness	Male	1,625	48.47	49.82	7.88	0.005	339	48.82	48.49	0.11	0.739
	Female	1,755	50.98	50.41	1.50	0.220	318	50.65	51.11	0.19	0.667
Openness to experience	Male	1,625	49.78	49.82	0.02	0.899	339	51.11	50.87	0.07	0.799
	Female	1,755	51.09	50.63	0.98	0.321	318	49.49	50.59	1.22	0.271
Imagination	Male	1,225	49.17	49.57	0.53	0.468	169	50.22	49.95	0.04	0.835
	Female	1,349	51.08	50.85	0.19	0.664	150	50.04	51.92	2.27	0.136
Intellect	Male	1,225	50.85	49.92	2.95	0.086	169	52.55	51.24	0.95	0.332
	Female	1,349	51.36	50.04	6.43	0.011	150	50.43	49.53	0.60	0.442
Intelligence (IQ)	Male	739	102.05	99.28	6.95	0.009	224	99.89	97.28	3.93	0.050
	Female	796	100.16	101.39	1.46	0.227	214	101.68	98.64	6.09	0.015

The NCDS sample could not be included in this analysis because it was missing information on siblings’ sex. We did not present results for families with one male and one female sibling because birth-order effects would be confounded with gender effects in these analyses.