E-cigarette use and onset of first cigarette smoking among adolescents: An empirical test of the 'common liability' theory.

E-cigarettes have become the most commonly used tobacco products among youth in the United States (US) recently. It is not clear whether there is a causal relationship between e-cigarette use and the onset of cigarette smoking. The "common liability" theory postulates that the association between e-cigarette use and cigarette smoking can be attributed to a common risk construct of using tobacco products. This study aims to investigate the relationship between ever e-cigarette use and cigarette smoking onset in the US using a structural equation modeling approach guided by the "common liability" theory. The study population is non-institutionalized civilian adolescents living in the US, sampled in the longitudinal Population Assessment of Tobacco and Health study. Information about tobacco product use was obtained via confidential self-report. A structural equation modeling approach was used to estimate the relationship between e-cigarette use at wave 1 and the onset of cigarette smoking at wave 2 after controlling for a latent construct representing a "common liability to use tobacco products." After controlling for a latent construct representing a "common liability to use tobacco products", ever e-cigarette use does not predict the onset of cigarette smoking (β=0.10, 95% CI= -0.09, 0.29, p=0.299). The latent "common liability to use tobacco products" is a robust predictor for the onset of cigarette smoking (β=0.42; 95% CI=0.08, 0.76; p=0.015). Findings from this study provide supportive evidence for the 'common liability' underlying observed associations between e-cigarette use and smoking onset.