. Author manuscript; available in PMC: 2011 Nov 1.

Published in final edited form as: Dev Psychopathol. 2010 Nov;22(4):831–848. doi: 10.1017/S0954579410000490

Table 3.

Twin Intraclass Correlations for Nicotine Dependence (NicD) and Major Depressive Disorder (MDD) Symptoms by Zygosity and Age

				Male		Female
		MZ	DZ	MZ	DZ	MZ	DZ
NicD	15 years	.51^***	.49^***	.53^***	.45^***	.48^***	.51^***
	18 years	.60^***	.37^***	.58^***	.38^***	.60^***	.38^***
	21 years	.55^***	.21^**	.51^***	.11	.62^***	.32^***
MDD	15 years	.02	.23^***	.12^*	.13	−.08	.28^**
	18 years	.28^***	.20^**	.35^***	.12	.24^***	.22^**
	21 years	.28^***	.14^*	.28^***	.06	.25^***	.15^*

Notes. Male twins are above diagonal, and female twins are below diagonal. MDD=major depressive disorder. NicD=nicotine dependence. MZ=monozygotic. DZ=dizygotic.

< .05,

^**

<.01,

^***

<.001.