Table 1.

Summary statistics and twin comparisons

	Swedish sample				P	U.S. sample				P
	MZ twins		DZ twins			MZ twins		DZ twins
	Mean	SD	Mean	SD		Mean	SD	Mean	SD
Education	0.69	0.46	0.70	0.46	0.95	0.65	0.48	0.71	0.45	0.26
Female	0.78	0.41	0.83	0.38	0.35	0.80	0.40	0.81	0.40	0.91
Age	32.4	7.4	34.6	7.5	0.03	36.9	16.7	32.4	14.8	0.02
Trust	0.77	0.28	0.80	0.25	0.26	0.52	0.27	0.51	0.25	0.87
Trustworthiness	0.39	0.15	0.40	0.16	0.53	0.45	0.25	0.45	0.24	0.90
Cognitive ability	0.01	0.98	−0.04	1.05	0.64
Emotional stability	−0.02	1.00	0.07	1.01	0.43
Agreeableness	0.02	0.98	−0.05	1.07	0.54
Extraversion	0.01	0.99	−0.04	1.03	0.63
Conscientiousness	−0.00	1.00	0.02	0.99	0.85

Education assumes the value 1 if the individual attended at least some college. Trust is the fraction sent, and trustworthiness is the fraction returned in the trust game. Psychological measures were adjusted to have mean 0 and standard deviation 1 for the whole sample. Tests of equality take into account the correlation between subjects by using the GEE cluster-robust variance–covariance matrix of Liang and Zeger (14). These data show that MZ and DZ twins differ significantly only in age, suggesting that MZ and DZ twins are drawn from very similar environments. To ensure that age is not a factor in the Swedish and U.S. experiments, we analyzed separate models for above- and below-median age subjects, and we find no significant differences in the estimates.