Oh et al. BMC Public Health 2010, 10:74 http://www.biomedcentral.com/1471-2458/10/74 findings on socio-economic status and smoking initiation could have also been due to generational dif- ferences between the younger and older women. In younger women, less education has been found to be associated with ever smoking; however, education has been found to be less predictive for older age groups. The generational shift varies in different European coun- tries, with larger inequalities in Northern Europe, and this could have also diluted our overall results [42-47]. Alternatively, the association between socioeco- nomic measures and ever smoking could in fact be accurate. Most studies investigating smoking initiation and socioeconomic factors look at populations that include both men and women. Women, however, may be less influenced by education and income status than men and more influenced by social factors such as friends and family. Many studies looking at SES and smoking status measure SES at the neighborhood level [14,15,36,38]. Using neighborhood level data, rather than individual level data, may account for societal fac- tors that may have a strong influence on women. One study showed that higher levels of smoking in deprived areas can only be partially explained by individual SES [15]. Similarly, a study based in California found that the protective effects of individual SES may be reduced if an individual lives in a low SES neighborhood [36]. Further investigation may provide interesting insight into what specific aspects of SES influence smoking in women. As has been seen elsewhere [6-12], friends smoking was the top reason given for smoking initiation. The second most common reason for initiation was to ‘look cool’. Spijkerman and colleagues also found that smok- ing was associated with wanting to be cool, rebellious and attractive [48]. Watson and colleagues found that smoking was accepted as part of a ‘cool’ image [49]. Friends smoking and looking cool was especially impor- tant for the younger women in our study. Over 80% of the smokers in our study started smoking by 20 years of age. Those who initiate smoking in ado- lescence are of particular concern. Adolescents are more susceptible to nicotine addiction and require a shorter duration of smoking and fewer cigarettes to become addicted [50]. Furthermore, the earlier one starts smok- ing, the greater the cancer risk in middle and old age [50,51]. Average age at smoking initiation varied by country, income, education, and age cohort. Schulze and Mons also found mean age of smoking initiation dif- fered by education level in Germany, and they found that the gap was widening among women [52]. Consis- tent with our findings, La Vecchia et al. found that Ita- lian women are initiating smoking at an earlier age than previous generations [53]. Differences in age at initiation Page 8 of 11 by country may be reflected in inter-country variation in lung cancer mortality by age [3]. We found depression to be associated with later smoking initiation. Since median age of onset of mood disorders, including major depressive disorder, ranges from 29 to 43 globally [54], it is not surprising that women who initiated smoking at a later age cited depression as a factor. In our study sample, 7.3% of women who started smoking at age 29 and older cited depression as a reason for initiating smoking compared to 1.2% in women who started smoking younger than 29 (p < 0.0001, data not shown). There is already much evidence linking depression and smoking [55,56]; how- ever, there is some debate as to the direction of the causality [57-61]. However, since depression was only evaluated by self- report, the definition of depression may have been used very broadly by the participants and it is difficult to make any specific clinical conclusions. It may be inter- esting for future studies to investigate if the relationship between depression and smoking differs by age at smok- ing initiation. We found that individuals who stated they began smoking to reduce stress were more likely to have initiated at an older age. Other studies have found an association between stress and smoking [62-64]. Lloyd and Taylor found that the effect of former life stress predicts smoking independently of recent stress and that the effect of social stress on smoking is additive over time [65]. In other words, stress at a younger age may affect future smoking; therefore, women who initiated smoking at a later age were more likely to cite stress as a factor. One major limitation of the study was the stratified sampling approach using available telephone numbers. This method was chosen to allow the study to reach a large sample of women that was proportionally repre- sentative of smoking rates in each age group. However, administering the survey via telephone prevented us from verifying self-reported data. The potential for recall bias on reported age of initiation may have affected the accuracy of results. This bias was not likely to be differ- ential by age group and thus should not affect the over- all picture of the results. Also, since no mobile phone numbers were included in the phone list, the study could have excluded a substantial number of women from our study who may have unknown differences than those who could be reached by home telephone. Level of income and/or education may have affected whether women had a landline or whether they relied solely on cellular phone use. Thus, the participants may not be an accurate representation of their respective countries.