2.3 percent being members of a twin pair and 12.5 percent of respondents with a twin pair in their family. They were then asked if it would be okay for the Harvard Medical School to contact them and their twin to participate in the study. The sample was limited to twins between the ages of 25 and 74, both with a residential telephone number, living in the United States and English-speaking, and both mentally and physically able to participate in the interview themselves. Roughly one-half of the first contacts were not eligible to participate in the study (mostly because they were too young). Sixty percent of the eligible contacts participated in the study and 21 percent of those who were referred by the first contacts participated, with an overall participation of 26 percent of the identified twin pairs. Overall, a total of 998 adult twin pairs were used in this study. We use only monozygotic (MZ) and same-sex dizygotic (DZ) pairs, dropping pairs with missing information about smoking history or age, and opposite sex DZ pairs. Our final analyses use a total of 340 MZ pairs and 315 same-sex DZ pairs.
