General Objectives

In the initial phase, the Turkish twin study aimed to investigate genetic and environmental factors affecting individuals’ smoking status and related phenotypes, and to determine the risks for smoking behavior of Turkish twins in the context of cultural differences. Smoking is a widespread habit and a major public health problem in Turkey. Cardiovascular diseases and cancer are the top two causes of mortality in Turkey. The World Health Organization's (WHO) 2011 Global Adult Tobacco Survey estimated regular smoking rates in Turkey as 31.1% among all individuals (males 47.8%, females 15.1%; WHO, 2011). Eighty-seven percent of deaths from lung cancer and about 30% of other cancer-related deaths are caused by smoking in developed countries (Bozkurt et al., Reference Bozkurt, Şahinöz, Özçırpıcı, Özgür, Şahinöz, Acemoğlu and Akkafa2006). The twin study was carried out at Kirikkale University, in Kirikkale, Turkey. Kirikkale is a city in the central Anatolian region of Turkey. It is located 80 km east of Ankara, the capital of Turkey. Until now, there have been no published studies on the smoking status of Turkish twins; furthermore, no twin data had been collected for any other research purpose.

Recruitment Strategy

We used official birth records to identify approximately 1,200 potential participants. Due to limited start-up funds for this project, we were only able to contact 640 of these individuals, randomly selected from this larger list. There were 14 instances when only one individual from a twin pair was interviewed. Four individuals refused to participate in our research due to the security agreements in their jobs as federal employees; their twins were removed from our list as well. Our final sample included 618 twins between ages 15 to 69 living in Turkey. The primary method of data collection was a face-to-face interview using two interviewers to ensure data quality. Each twin was interviewed separately. Completion took an average of 25 minutes. Municipial authorities also helped interviewers in rural regions to prevent misunderstandings during the data collection. All twins were interviewed following agreement with their families, and afterwards the interviewers presented the participants with official documents and explained research purposes and data security issues. Almost all individuals agreed to participate in the future follow-up studies, which will include DNA collection.

Zygosity Diagnosis

Zygosity was determined based on responses to a standard zygosity questionnaire (Kaprio et al., Reference Kaprio, Sarna, Koskenvuo and Rantasalo1978).

Phenotypes

Participants responded to questions about age, gender, smoking status of his or her twin, smoking status and education levels of parents, income, daily sports activities, smoking history (age when started or quit smoking, daily average number of cigarettes smoked, attempts to quit smoking, the reasons for starting smoking), alcohol use, and behavioral problems. Answers to some of questions were determined by cross-checking twins’ reports when possible and were defined as missing data if the twins conflicted.

Sample Characteristics

We assessed 302 twin pairs and 14 single twins (330 males and 274 females, both smokers and non-smokers). The mean age of our twins was 25.93 ± 9.98, minimum 15, and maximum 69. Out of 302 pairs, 107 (35.4%) pairs were monozygotic (MZ), 186 (61.6%) pairs were dizygotic (DZ), and 9 (2.9%) pairs were of unknown zygosity.

It was difficult to identify the income level in Turkey because of the high inflation rate. Accordingly, income was coded 0 (income <US $1,200/month) or 1 (income ≥US $1,200/month). Parental education level was coded 0 (no school, primary school, secondary school, or middle school) or 1 (high school, university, or graduate school). According to a Turkish adaptation (Uysal et al., Reference Uysal, Kadakal, Karşıdağ, Bayram, Uysal and Yılmaz2004) of the Fagerström Test for Nicotine Dependence (Fagerström, Reference Fagerström1978), 178 participants (28.8%) were identified as smokers (smoking every day for a month or longer). Of these smokers, 79.2% were male and 20.8% were female. Table 1 provides frequency distributions of some measured variables by zygosity on an individual basis.

TABLE 1 Characteristics of the Study Population from a Twin Study, Turkey

* Includes both same-sex and opposite-sex dizygotic twins.

Future Directions

The proposed study is likely to provide valuable information regarding factors related to nicotine dependence, their relation to twin zygosity, gender, social situation, and family structure. We plan to compare our results with those from other well-known worldwide studies. The detailed analysis of these epidemiological data will provide important information about understanding nicotine dependence, which may inform efforts to prevent nicotine dependence. We plan to apply for research grants, recruit more subjects, and officially register the Turkish Twin Registry with the Turkish government.