Futhermore, one of the problems noted in the deeming regulations is the fact that: "The vast majority of the respondents who were aware of these products indicated that they believed e-cigarettes were less harmful than traditional cigarettes...".

Although the survey approach described above would lead to "invalid" results, I would never argue that survey studies should be thrown out and that we should only be conducting RCTs. My point is that there are strengths and limitations of various study designs, and causal inference is enhanced by having research evidence from multiple study designs. On balance, the strengths and weaknesses balance each other out, leading to a much greater ability to draw valid conclusions.

Fortunately, there is a simple way to evaluate this hypothesis, at least on a preliminary basis, prior to waiting for the results of longitudinal studies. Enough youth are experimenting with electronic cigarettes, and the increase in youth e-cigarette use has been dramatic and sustained enough, that if the gateway hypothesis were correct, we would be starting to see an increase in smoking prevalence among youth due to the many new smoking initiates that were converted from e-cigarette experimentation to smoking.

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