. Author manuscript; available in PMC: 2013 May 1.

Published in final edited form as: Behav Genet. 2011 Oct 30;42(3):345–353. doi: 10.1007/s10519-011-9516-8

Table 1.

Frequency of lifetime substance use and mean scores for family environment variables by zygosity

	Smoking (%)	Alcohol (%)	Marijuana (%)	Cocaine (%)	Stimulant (%)	Sedative (%)	Hallucinogen (%)	Any illicit drug (%)	Parental attitudes mean (SD)	Household use mean (SD)
MZ	85	94	58	20	21	12	17	59	3.41 (2.3)	6.74 (4.0)
DZ	87	96	64	24	24	15	20	65	3.63 (2.3)	7.10 (4.1)

Frequency of lifetime substance use indicates the percentage of those respondents ever having used the substance in their life SD standard deviation