TABLE 1.

Number of Twin and Sibling Pairs and the Tetrachoric Correlation for and Prevalence of Drug Abuse in these Pairs^a

Sex	Pair type	Number of Complete Pairs	Number of Concordant Pairs	Number of Discordant Pairs	Tetrachoric Correlation	Standard Error	Prevalence of Drug Abuse (%)
							Male	Female
Male-Male	Monozygotic twins	3,899	47	122	0.79	0.03	2.8
Male-Male	Dizygotic twins	4,238	24	157	0.58	0.06	2.7
Female-Female	Monozygotic twins	4,558	20	98	0.70	0.05		1.5
Female-Female	Dizygotic twins	4,313	8	110	0.45	0.09		1.5
Male-Female	Dizygotic twins	12,447	45	663	0.36	0.05	4.0	2.1
Male-Male	Full siblings	718,276	7,792	49,908	0.51	0.00	4.6
Female-Female	Full siblings	641,506	1,139	20,133	0.37	0.01		1.8
Male-Female	Full siblings	1,347,324	4,063	76,392	0.32	0.01	4.3	1.7

^aFor full sibling pairs, we took all pairs within sibships up to four. With larger sibships, we picked four pairs at random.