Lifestyle factors including cigarette smoking, obesity, and alcohol consumption are responsible for 40-60% of cancers in Western countries. We want to better understand the effect of these factors – as well as of mental health problems such as depression, stress, and anxiety – on smoking behaviors in adolescents and young adults. In particular, we are interested in how these factors influence the onset, development, and cessation of cigarette smoking. We will identify the causal associations between initiation of alcohol and cannabis use on these smoking outcomes. By gaining better understanding of why adolescents and young adults begin, continue, and stop smoking cigarettes, we can inform the development of more effective preventive interventions, and ultimately reduce the incidence of cancer later in life.