Table 3.

Observed (phenotypic) correlations, cross-trait/cross-twin correlations, and genetic correlations

Left hemisphere, IQ (x, y, z)	Correlation				Right hemisphere, IQ (x, y, z)	Correlation
	Observed (rp)	Cross-trait/cross-twin		Genetic (rg)		Observed (rp)	Cross-trait/cross-twin		Genetic (rg)
		MZ	DZ				MZ	DZ
Gray matter					Gray matter
Occipital cortex, PIQ (−21, −62, 13)	0.17	0.08	−0.06	0.23	Medial frontal, PIQ (41, 22, 43)	0.25	0.32	0.02	0.37
White matter					Parahippocampal, PIQ (23, −36, −9)	0.23	0.24	−0.04	0.40
Superior occipitofrontal fascicle, PIQ (−21, −24, 35)	0.17	0.18	0.04	0.24	Parahippocampal, VIQ (23, −38, −7)	0.11	0.15	−0.02	0.19
Corpus callosum, PIQ (−15, 3, 33)	0.18	0.17	0.17	0.22	White matter
Optic radiation, PIQ (−35, −48, 17)	0.26	0.28	0.01	0.39	Superior occipitofrontal fascicle, PIQ (21, −29, 31)	0.35	0.25	0.08	0.32
					Corpus callosum, VIQ (15, 3, 37)	0.14	0.13	0.02	0.15

The observed correlation (r_p) reflects the correlation that is found between GM and WM density and VIQ/PIQ within individuals. The cross-trait/cross-twin correlations show the correlations of focal GM and WM densities of a twin with the intelligence quotient of his/her cotwin and vice versa for MZ and DZ twins (or sibling pairs) separately. If the correlation between GM and WM density of a twin and VIQ or PIQ in the cotwin is larger in MZ than in DZ twins, this indicates that the genes influencing GM and WM density partly overlap with the genes that influence VIQ/PIQ. The extent of the overlap is reflected by the magnitude of the genetic correlation (r_g). r_g reflects the correlation between the set of genes that influences focal GM and WM density and the set of genes that influences VIQ/PIQ. r_{p =} r_gh₁h₂ + r_ee₁e₂ (i.e., the phenotypic correlation is r_g weighted by the square roots of the heritabilities h₁ for GM and WM density and h₂ for VIQ/PIQ plus the unique environmental component r_ee₁e₂).