on drug policy and legislation in many countries. The gateway hypothesis offers one possible
explanation for young people's development of a serious drug problem. It simply states that
the use of one drug increases the risk of starting to consume another, and possibly more
harmful, drug later on and that the risk increases with frequency of use (dose-response). The
empirical basis for the hypothesis is the common finding that most heavy drug users have
started with less dangerous drugs first and that there seems to be a "staircase" from alcohol
and insolvents via cannabis and tablets to amphetamine, cocaine and heroin. The core
question is whether the sequential initiation pattern of drug use is best explained by the
mechanisms substantiating the gateway hypothesis or whether the phenomenon is better
understood by employing the concepts of accessibility and/or transition proneness? Based on
a representative sample of 21-31 year olds in Oslo we have examined the possible gateway
effect of both legal (alcohol) and illegal drugs (cannabis) on subsequent use of cannabis and
hard drugs (amphetamine and cocaine). We use multivariate probit models that take account
of unobservable individual-specific effects to reduce the possibility of a spurious causal effect
of soft drug use on the onset of hard drug use. The gateway effects were greater when we did
not take account of unobserved heterogeneity, but, although substantially reduced, they
remained considerable also when unobserved factors were accounted for.