Table 1.

Parameter Estimates From Univariate and Bivariate Twin Models.

	Univariate Models								Bivariate Models (Interest → Achievement)
	Achievement				Interest
	1. Main Effects		2. Interaction		3. Main Effects		4. Interaction		5. Main Effects		6. Interaction		7. Final Model
Parameter	Estimate	SE	Estimate	SE	Estimate	SE	Estimate	SE	Estimate	SE	Estimate	SE	Estimate	SE
a i					0.523	0.110	0.503	0.127	0.538	0.106	0.490	0.124	0.536	0.095
ai'							0.101	0.068			0.111	0.068
ci					0.459	0.113	0.460	0.123	0.445	0.117	0.470	0.109	0.447	0.104
ci'							−0.100	0.083			−0.103	0.07
ei					0.700	0.022	0.699	0.023	0.698	0.022	0.700	0.022	0.699	0.021
ei'							−0.037	0.020			− 0.04	0.02
a b									0.274	0.107	0.283	0.105	0.281	0.086
ab'											0.135	0.068	0.122	0.043
cb									0.412	0.138	0.381	0.108	0.408	0.106
cb''											0.014	0.08
e b									0.061	0.016	0.057	0.016	0.059	0.016
eb'											−0.023	0.015
a a	0.668	0.044	0.655	0.045					0.609	0.056	0.586	0.055	0.589	0.055
aa'			0.120	0.046							0.055	0.045
c a	0.596	0.055	0.592	0.059					0.430	0.130	0.447	0.098	0.441	0.103
ca'			−-0.079	0.064							−0.103	0.079
e a	0.359	0.012	0.358	0.012					0.353	0.011	0.352	0.011	0.353	0.011
ea'			−0.012	0.011							−0.010	0.011
s i					.127	.030	.127	.031	.130	.030	.135	.031	.135	.030
s a	.243	.032	.251	.034					.242	.032	.253	.034	.253	.034

Note. Parameters in bold are significant at P <.05.