. Author manuscript; available in PMC: 2014 Dec 1.

Published in final edited form as: Drug Alcohol Depend. 2013 Aug 11;133(2):10.1016/j.drugalcdep.2013.07.034. doi: 10.1016/j.drugalcdep.2013.07.034

Table 2.

Tetrachoric twin pair correlations (and 95% confidence intervals) for cannabis use and early school leaving by zygosity.

Zygosity	N pairs	Cannabis use before the age of 18		Early school leaving
Zygosity	N pairs	Tetrachoric twin pair r (95% CI)	MZ versus DZ Twin pair r (95% CI)	Tetrachoric twin pair r (95% CI)	MZ versus DZ twin pair r (95% CI)
MZ F	399	0.80 (0.71; 0.87)	0.77 (0.69; 0.83)	0.76 (0.59; 0.88)	0.76 (0.62; 0.86)
MZ M	172	0.71 (0.55; 0.83)	0.77 (0.69; 0.83)	0.76 (0.47; 0.91)	0.76 (0.62; 0.86)
DZ F	302	0.58 (0.42; 0.71)	0.46 (0.35 – 0.56)	0.64 (0.41; 0.80)	0.60 (0.44; 0.72)
DZ M	120	0.31 (0.03; 0.55)		0.52 (0.09; 0.80)
DZ OS	231	0.38 (0.18; 0.56)		0.58 (0.31; 0.77)

F=female, M=male, OS=opposite-sex, MZ =monozygotic, DZ=dizygotic, CI= confidence interval, MZ versus DZ =pooled MZ and DZ twin pair correlations (i.e. equated across sexes).