A Dual-Process Discrete-Time Survival Analysis Model: Application to the Gateway Drug Hypothesis

The gateway drug model is a popular conceptualization of a progression most substance-users are hypothesized to follow as they try different legal and illegal drugs. Most forms of the gateway hypothesis are that "softer" drugs lead to "harder," illicit drugs. However, the gateway hypothesis has been notably difficult to directly test - i.e., to test as competing hypotheses in a single model that licit drug use might lead to illicit drug use or the reverse. This article presents a novel statistical technique, dual-process discrete-time survival analysis, which enables this comparison. This method uses mixture-modeling software to estimate two concurrent time-to-event processes and their effects on each other. Using this method, support for the gateway hypothesis in the National Longitudinal Survey of Youth 1997 was weak. However, this paper was not designed as a strong test of causal direction but more as a technical demonstration, and suffered from certain technological limitations. Both these limitations and future directions are discussed.