Table 2.

Number of Twins Assessed for Smoking Initiation (SI) Across Age and Sample (Top), Including Number of Individual Assessments (IA), Number of Unique Individual Twins per Sample (UI) Between the Ages of 10 and 19, and by Zygosity and Sample (Bottom), Including Number of Unique Individuals (UIZ) and Pairs of Twins (UPZ) With Known Zygosity per Sample

Age	10	11	12	13	14	15	16	17	18	19	IA	UI
US			86	161	346	442	491	448	295	251	2520	1480
CO			401	363	418	321	298	958	528	510	3797	2770
MI		646	1763	105	535	1629	383	1024	1979	474	8538	4128
NC		164	335	404	382	277	276	211	122		2171	2172
VA	565	636	619	755	884	755	829	660		183	5886	2710
VA2	476	826	178	712	542	40	434	305	72		3585	1148
BE	124	187	187	186	186	196	181	114	181		1542	210
FI					4709		5729	10 683	6490	1133	28 744	13 262
NL		10	132	494	993	1090	1410	1536	2089	1944	9698	6036
SW				853	1352	43	924	1395	41	672	5280	2561
AU		112	328	329	507	496	956	761	709	399	4597	2869
Total	1165	2581	4029	4362	10 854	5289	11 911	18 095	12 506	5566	76 358	39 346
	MZm	MZf	DZm	DZf	DZo	UIZ	MZm	MZf	DZm	DZf	DZo	UPZ
US	268	282	246	220	380	1396	138	143	127	113	201	722
CO	613	729	440	457	529	2768	309	365	222	229	266	1391
MI	1166	1226	642	728	0	3762	583	613	321	364	0	1881
NC	329	560	283	393	497	2062	179	301	159	209	286	1134
VA	623	805	345	370	559	2702	312	403	173	185	281	1354
VA2	288	308	146	164	242	1148	144	154	73	82	121	574
BE	42	44	42	40	42	210	21	22	21	20	21	105
FI	1828	2241	2436	2393	3542	12 440	919	1125	1235	1208	1775	6262
NL	939	1535	793	1009	1646	5922	494	816	421	551	913	3195
SW	475	515	337	388	754	2469	241	258	170	195	387	1251
AU	574	751	395	441	704	2865	288	376	199	225	356	1444
Total	7145	8996	6105	6603	8895	37 744	3628	4576	3121	3381	4607	19 313

MZm = monozygotic male; MZf = monozygotic female; DZm = dizygotic male; DZf = dizygotic female; DZo = dizygotic opposite sex.