Chapter 8 DRUG USE AMONG THE SPANISH ADOLESCENTS: TESTING FOR A CAUSAL GATEWAY EFFECT

The use of drugs represents a particularly worrying public health problem among adolescents. In this article we present evidence in favour of the causal gateway effect in the use of drugs among the Spanish adolescent population. We use instrumental variables in order to consistently estimate two-stage probit models from the information provided by three waves of the Spanish Surveys on Drug Use in the School Population (1996, 1998 and 2000). After treating the identification problem by means of the natural experimentation methodology, our results indicate that smoking cigarettes does indeed causally induce the use of marijuana with such use, in turn, increasing the probability of the adolescent taking the ultimate step, that is to say, hard drug use. We also provide evidence which confirms the herd effect among this age-group as a reason for starting to consume tobacco and marijuana, although not for the use of hard drugs. As a conclusion, not only does the observed gateway effect in drug use among Spanish adolescents correspond to a spurious correlation, but also to a causal mechanism