Author's reply

We thank Abraham et al for their comments, but they have misrepresented our paper, and we find their arguments either misleading or unconvincing.

Abraham et al complain that 16 of our 28 statistical comparisons contrast a Dutch city with a national estimate from the USA or another nation, suggesting that we “pre-suppose that prevalence rates are the same all over The Netherlands”. We made no such presupposition. As we clearly stated in our article: “American surveys indicate little difference, on average, between large metropolitan samples and the USA as a whole… but the estimates in Table 1 suggest that Amsterdam has a higher fraction of marijuana users than smaller Dutch communities. US rates are basically identical to those in Amsterdam and Utrecht, and higher than those Tilburg”. We then note that “unfortunately, many of the available contrasts between The Netherlands and her European neighbours suffer from the same weakness, comparing rates for an entire nation as a whole to those in the largest city of another nation”. And we state that the contrasts where the Dutch rates are higher are mostly “attributable to comparisons limited to Amsterdam”. We conclude that “Dutch rates are somewhat lower than those of the USA but somewhat higher than those of some, but not all, of its neighbours. Amsterdam's level of marijuana use is comparable to that of the USA”.

Abraham et al further complain that our comparisons were “ arbitrarily selected”. In fact, our 1997 Science article included every Dutch cannabis prevalence rate for which we could find a reasonable international contrast matched by year, age range and type of prevalence. Our recent update in the British Journal of Psychiatry added another 13 comparisons. We welcome further comparisons but a fair reading of both papers makes it clear that we attempted to be exhaustive, given the limited availability of Dutch drug prevalence data in English-language sources. (Indeed, where possible we had Dutch-language sources translated.) In any case, we emphasise that we drew no policy conclusions from these static cross-sectional comparisons. That portion of our article was an attempt to correct grossly misleading comparisons of Dutch and US rates in the American media.

We are taken to task for using the Dutch school survey data from the Trimbos Institute, rather than data from Statistics Netherlands or the CEDRO Amsterdam survey. As noted below, we did in fact report CEDRO estimates. But the 1990s Amsterdam trends mentioned by Abraham et al are not relevant to our commercialisation thesis; as we explained in our article, the dramatic growth in cannabis commercialisation in Amsterdam occurred between 1980 and 1988 and almost every Western nation saw increases in cannabis use after 1992 for reasons apparently unrelated to drug policy.

We are delighted to learn of the national Statistics Netherlands estimates, which as far as we can tell have not been cited previously in the English-language literature — although the search engine on their website produces no statistics for “drugs”, “drug”, “ cannabis” or “marijuana”. But now we are puzzled as to why a 1997 paper by Marieke Langemeijer announcing CEDRO's own national survey stated that “The implementation of the national survey means that finally, The Netherlands will have a decent source of data that serves multiple purposes among which the basic information for health care, prevention, education and drug policy. Hopefully, it is the beginning of a high quality drug research tradition”. Similarly, a CEDRO press release of 14 April 1998 stated that “figures for the entire country will soon no longer have to be based on local surveys since a national study on drug use in The Netherlands is currently being carried out by CEDRO”. Moreover, neither the CEDRO nor the Trimbos researchers mention these data in their English-language monographs on Dutch drug use trends.

Our Fig. 1 showed that during the 1984-1992 period the Trimbos lifetime prevalence estimates rose even more steeply for the age 16-17 group than for the age 18-20 group. This clearly undermines the concern raised by Abrahamet al about a selection bias involving older students, but at any rate, that criticism misses the point. Sampling biases of the Trimbos school survey do not preclude its use for studying trends over time. Moreover, our trend analysis compared it to age 18-20 trends from the US Monitoring the Future school survey. The Trimbos researchers state that their survey was designed to permit comparisons to that US survey (see Plomp et al, 1991: 11).

Abraham et al complain that we averaged non-comparable estimates, but fail to mention that we grouped our estimates so that ‘cityv. nation’ averages and ‘nation v. nation’ averages were presented separately. We think our averaging was well within contemporary standards of meta-analysis, but no matter — we presented the raw data so readers could decide for themselves. At any rate, no conclusions of our work hinged on these averages — indeed, we did not even include them in our presentation of these data in our forthcoming book, Drug War Heresies (MacCoun & Reuter, 2001b).

Abraham et al suggest that our alleged inattention to the geographical issue undermined our inferences about the effects of commercialisation. On the contrary, the fact that cannabis prevalence is higher in Amsterdam is quite consistent with our hypothesis. During the 1980s, when we contend the commercialisation effect occurred, various estimates suggest that over a quarter of all Dutch cannabis coffee shops were in Amsterdam, yet Amsterdam accounted for only about 5% of the total Dutch population. As late as 1997, Abraham et al (1999) reported that last-year users from the highest-density Dutch addresses were more likely to cite coffee shops as their cannabis source than were users from low-density Dutch addresses.

As we stated in the article, the evidence for our commercialisation hypothesis was indirect and at best purely correlational, though we noted that it is consistent with evidence on gambling, tobacco and alcohol marketing. Moreover, the quasi-legal status of the Dutch system, which tends to keep prices high, almost surely understates the likely commercialisation effects of full legalisation. Given weak data, our inferences may well be wrong, but we think the comments of Abraham et al shed little light on that question.