Significance of included SNPs
Although p < 5*10-8 has long been widely accepted as a threshold for genome-wide significance in GWASs,\cite{Fadista_2016} the p-values of the SNPs included in the catalog varied widely, and many were above this threshold, implying that they are not genome-wide significant hits. The largest p-value for the 2,719 SNPs included here was 9*10-6, and the smallest was 7*10-39. The average p-value was 1.02*10-6.
In total, 1,728 of the 2,719 SNPs (63.6%) had p-values below 5*10-8, and thus could be considered genome-wide significant. This can also be viewed from the perspective of the unique SNPs, rather than the total number of SNPs, in which case, of the 2,047 SNPs reported only once, their single p-value was significant for 1,155 (56.4%) of them.
It has previously been reported that the distribution of p-values tends to follow an exponential decreasing trend.\cite{Masicampo_2012} In the current dataset, similar evidence of an exponential decreasing trend was found when the distribution of p-values was examined, as I discuss below.
First, I compiled all negative logs of p-values from the original downloaded table (which were found in the column "PVALUE_MLOG"). Then I generated a histogram based on the frequency of their occurrence. The minimum -log(p) value in the table was 5.045, and the maximum was 38.15 (both approximate). Therefore the "bins" I chose for the histogram ranged from 5 to 41, at intervals of 3.
Replication samples
Recall that a total of 17 studies are included in this review, but only 14 of them reported any associations whatsoever. Of these 17 studies, 4 of them (24%) followed up their initial set of associated SNPs in a subsequent replication sample. Of these 4, 3 (75%) did not replicate any of the SNPs they had initially found to be associated with intelligence, meaning they reported no associations at all. The single exception was Benyamin et al. (2014), who followed up their initial GWAS hits in their discovery sample (n = 12,441) with another GWAS in a replication sample (n = 5,548). According to the catalog, these authors reported six SNPs as associated with intelligence,\cite{catalog} but acknowledged that "no individual single-nucleotide polymorphisms (SNPs) were detected with genome-wide significance".\cite{Benyamin2014} Further examination of the catalog entry for this study shows that the p-values of the six SNPs listed as associations are all below 5*10-8 (specifically, all of them are between 5 and 9*10-6). In summary, it appears that every study analyzed here which included a replication sample was unable to find any genome-wide significant associations in it.
Temporal trends
Diversity
An abject lack of diversity was also evident in regard to the populations on which the studies were conducted: all 13 (100%) of them that reported ancestry (or ancestries) stated that all of their participants were of European ancestry (this includes "British ancestry"). The only exceptions were one study which only reported ancestry information for one of the two samples they analyzed,\cite{Hill2019} and another study which did not report any ancestry information at all.\cite{Gialluisi2014}