. Author manuscript; available in PMC: 2015 Nov 4.

Published in final edited form as: Twin Res Hum Genet. 2015 Jul 31;18(5):497–506. doi: 10.1017/thg.2015.48

Table III.

Polychoric Correlations for Cigarette Quantity (95% CI) by Sex, Zygosity and Age Group

	Females		Males		DZO
	MZ	DZ	MZ	DZ	DZO
14-15 year-olds	0.47	-0.23	-0.17	0.04	0.61
(n=287 twin pairs)	(0.23, 0.71)	(-0.54, 0.08)	(-0.50, 0.15)	(-0.21, 0.29)	(0.42, 0.81)
16-17 year-olds	0.76	0.47	0.33	0.26	0.55
(n=389 twin pairs)	(0.63, 0.88)	(0.26, 0.68)	(0.13, 0.54)	(0.07, 0.45)	(0.40, 0.70)
18-25 year-olds	0.61	0.24	0.55	0.08	0.24
(n=709 twin pairs)	(0.49, 0.73)	(0.04, 0.44)	(0.42, 0.68)	(-0.10, 0.25)	(0.07, 0.40)
26-33 year-olds	0.59	0.00	0.69	0.00	0.18
(n=723 twin pairs)	(0.46, 0.72)	(-0.23, 0.23)	(0.57, 0.80)	(-0.18, 0.17)	(0.11, 0.25)

Note: Non-smokers were not included in the correlations presented here for cigarette quantity. If the liabilities to cigarette quantity are independent of one another then the twin correlations for cigarette quantity will accurately reflect the findings from the causal-contingent-common pathway model, otherwise, if the liabilities are dependent, they will not.