Table 2.

Twin Intraclass and Cross-Correlations for Hyperactivity and Achievement

	Males		Females
Intraclass correlations	MZ, r	DZ, r	MZ, r	DZ, r
Unadjusted achievement^a	.85^**	.47^**	.80^**	.52^**
Achievement	.77^**	.47^**	.72^**	.51^**
Hyperactivity
Parent ratings	.57^**	−.04	.55^**	−.08
Same teacher rating	.76^**	.38^**	.72^**	.28^**
Different teacher rating	.64^**	.20 ^*	.49^**	.20 ^*
Twin cross-correlations

Hyperactivity × Unadjusted Achievement
Parent ratings	−.28^**	−.07	−.21^**	−.08
Same teacher rating	−.35^**	−.25^**	−.37^**	−.22^**
Different teacher rating	−.34^**	−.17^*	−.40^**	−.18^*
Hyperactivity × Achievement
Parent ratings	−.24^**	−.07	−.18^**	−.06
Same teacher rating	−.34^**	−.22^**	−.33^**	−.19^**
Different teacher rating	−.28^**	−.16^*	−.33^**	−.20^*

Note. Number of twin pairs, parent ratings: MZ males = 405, DZ males = 381, MZ females = 475, DZ females = 427; same teacher ratings: MZ males = 281, DZ males = 271, MZ females = 334, DZ females = 311; different teacher ratings: MZ males = 164, DZ males = 152, MZ females = 177, DZ females = 146.

Scores were not residualized for general cognitive ability.

p<.05;

^**

p<.01.