More surprisingly, the scientific literature is no more critical. Between its publication and February 2010 the study by Volkow et al (2007) has been cited 30 times in scientific articles. Among them, 20 articles cited the conclusion that dopamine activity is depressed in ADHD without further comment. Apart from our review article [11] , none of them pointed out its internal inconsistency.

The point of interest here is that both articles have been echoed in the media as shown in Table 1 . The media almost always reported on the claimed conclusion. Indeed, concerning the article by Volkow et al (2007), we have checked 40 media articles and the conclusion that dopamine is depressed in the brain of ADHD patients has been always reported. We have never read a mitigating statement saying that their results are open to the opposite interpretation although the authors explicitly raised this possibility in their result section ( Table 1 ). In our sample of 21 articles that reported on the study by Barbaresi et al (2007) in the media, only one (The Guardian, London, September 21, 2007) adequately described the results and, thus questioned the conclusion claimed by Barbaresi's group ( Table 1 ).

In our review of the ADHD literature [11] , we have read about 360 articles and we have found only two studies showing obvious discrepancies between results and claimed conclusions [12] , [13] . These internal inconsistencies have already been discussed in detail [11] and are summarized in Table 1 . Our observation that only two articles among 360 show obvious internal inconsistencies must be considered with caution however. First, our review of the ADHD literature was not a systematic one and was not aimed at pointing out internal inconsistencies. Second, generalization to other fields of the neuroscience literature would be unjustified. We can only say that our observations confirm our intuition: this first type of misrepresentation is, fortunately, infrequent.

Fact omission

This misrepresentation consists of putting in the summary a fixed conclusion while raw data, which strongly limit the relevance of this conclusion, are only given in the result section. To quantify this misrepresentation, we have extensively studied how the scientific literature reports on a specific issue: the association between alleles of the gene coding for the D4 dopamine receptor (DRD4) and ADHD.

To fully appraise this misrepresentation it is illuminating to compare a statement in the media and the corresponding facts. The health guide of the New York Times says: “Genetic factors may play the most important role in ADHD…. Most of the research on the underlying genetic mechanisms targets the neurotransmitter dopamine. Variations in genes that regulate specific dopamine receptors have been identified in a high proportion of people with ADHD.” Actually, “the most robust finding in ADHD is the association of a variable number tandem repeat polymorphism in exon 3 of the DRD4 gene” [14]. However, although the 7-repeat allele is significantly associated with ADHD, it confers small risk [15]: ADHD patients have a higher frequency of this allele as compared to controls, 23% versus 17%, respectively [16]. Therefore, there is a huge gap between the media statement and the neurobiological facts. This gap is generated when scientific texts report the association of the 7-repeat allele of the DRD4 gene with ADHD but do not mention at the same time that it confers small risk.

To quantify this misrepresentation in the scientific literature, we examined the summaries of all 219 articles about ADHD that mentioned the DRD4 gene. Articles were classified between review articles (52), animal (or in vitro) studies (24) and research articles in humans (143). This third category was further divided into articles, in which genetic data related to the DRD4 were provided (117) or not (26). In this second category, as well as in animal studies, statements related to the association of the DRD4 gene with ADHD thus corresponded to citations of other articles. In these 219 summaries we counted the presence of specific statements as indicated in Table 2.

Among the 117 primary studies in humans, 74 articles state in their summary that alleles of the DRD4 genes are significantly associated with ADHD but only 19 summaries also mentioned that they confer a small risk. One may argue that summaries are too short to report the details. However, almost the same number of summaries (14) did not mention that it confers small risk but reinforced the view that genetic factors play the most important role in ADHD with an additional statement about its high heritability. Moreover, this misrepresentation always occurs in the summaries of primary articles that cite the association of the DRD4 gene with ADHD but do not report data on it (Table 2).

This misrepresentation is even more robust in review articles. Among the 43 relevant summaries stating that the DRD4 gene is significantly associated with ADHD only 6 mentioned that the 7-repeat allele confers a small risk. Again one may argue that this is due to length constraints, but this explanation is not consistent with other observations. Indeed, 13 summaries did not mention that it confers a small risk but added a statement on the high heritability of ADHD. Likewise, 9 summaries also mentioned the following type of erroneous statement: “The efficacy of stimulant agents confirms that the neurotransmitter abnormalities seen in ADHD are primarily catecholaminergic in origin.” The weakness of this argument has long been underlined [11], [17], [18] and relies on the fact that psychostimulants enhance attention to the same extent both in ADHD and healthy children [17].

On the whole, the case of the association between ADHD and the DRD4 gene shows that the omission of relevant facts limiting the impact of the claim is not restricted to a few scientific articles: it occurs in a vast majority of the summaries. Although in most reports and review articles, the raw data (e.g. odds ratios) were given inside the results section, it is likely that many readers may not check inside the text the relevance of the statement put in the summary (“the DRD4 gene is associated with ADHD”).

This misrepresentation is also observed in media articles. Indeed, we looked for press articles reporting on the DRD4 gene and on ADHD. Among 170 relevant articles published from 1996 to 2009, all but 2 stated that polymorphisms of the DRD4 gene are significantly associated with ADHD. Twenty-five articles also mentioned either the raw data or that it confers small risk, while 117 articles did not. Furthermore, 26 articles mentioned the odds ratio (from 1.2 to 1.34) but also put an overstated conclusion (e.g. “These findings strongly implicate the involvement of brain dopamine systems in the pathogenesis of ADHD.”). Thus, the 26 equivocal articles being discarded, 82% of the media articles misrepresented the association between the DRD4 gene and ADHD. This omission rate is very similar to that observed in scientific articles (Table 2).

The literature on the association between the DRD4 gene and ADHD further exemplifies a major publication bias: the most robust effects are reported in initial studies [5]. Indeed, although this association is still considered to be highly statistically significant, its odds ratio decreased with successive studies from 2.4 in the oldest study in 1996 [19] to reach 1.27 in the most recent meta-analysis [15]. This decrease in the clinical relevance of this association is not correlated with parallel changes in type-2 misrepresentation. Indeed, omission rates both in scientific and media articles did not decrease over the years 1996 to 2009 (Table S1).