Lynn Zimmer, PhD, Professor Emeritus at Queens College at the City University of New York, stated in his 1997 book Marijuana Myths - Marijuana Facts:

“In the end, the gateway theory is not a theory at all.  It is a description of the typical sequence in which multiple-drug users initiate the use of high-prevalence and low-prevalence drugs.

A similar statistical relationship exists between other kinds of common and uncommon related activities.  For example, most people who ride a motorcycle (a fairly rare activity) have ridden a bicycle (a fairly common activity).  Indeed, the prevalence of motorcycle riding among people who have never ridden a bicycle is probably extremely low.  However, bicycle riding does not cause motorcycle riding, and increases in the former will not lead automatically to increases in the latter. 

Nor will increases in marijuana use lead automatically to increases in the use of cocaine or heroin.”