To better understand the UK data, I plotted the standardized difference by birth year, which I simply estimated as study publication date minus median subject ages. The studies presented by Lynn (2006) are in red; the studies that I have found are in blue. As for the latter, I had the guesstimate subject ages for a few samples. For the clinical and law aptitude tests, I assigned an age of 30, since in one of the data sets this was the median age of the applicants. For the industrial samples, I assigned an age of 40, since this is the median age of workers. I also correct the Situation Judgement Test difference up to 1, as in the US a difference of 0.5 on these tests corresponds to a cognitive differences of 1 (refer to Roth, et al. for the intercorrelations). As For the military samples, I assigned an age of 20, on the assumption that this was the median age of armed force applicants. I also had to estimate the standardized difference based on SAT scores, which I did by assuming a White standard deviation of 100. It should be noted that all of the studies were problematic in some way. They were either based on convenience samples (e.g., most of Lynn’s samples), based on unrepresentative samples (e.g., the clinical and law samples), unpublished (e.g., the gl-assessment sample), based on subtests scores and not full scale IQs (e.g., the MCS sample), and so on. Generally, the gaps shows a secular narrowing, but are not particularly consistent with the hypothesis that the cognitive ability gap has closed in the last generation.
I also plotted the standardized difference by test year. This, it can be seen, paints a somewhat different picture. This is due to my data points having a number of adult samples. Contemporaneous British Blacks age 30 and up, it seems, score around 0.8 standard deviations below Contemporaneous British Whites. An important unresolved question is whether or not the gap increases with age longitudinally as it does in the US. The narrowing shown when plotting data by birth date could represent a narrowing merely at young ages, for a given birth cohort, when the heritability of IQ is low, or it could represent a genuine narrowing of the population difference across age cohorts. The former would be consistent with a genetic hypothesis, while not the latter.
This then leaves a puzzle. The median birth year of the various achievement samples, which show trivial to small gaps, is 1992. (The kids were 14-16, median 15, and the tests were taken between 2004 and 2010, median 2007.) Based on the trend line from the cognitive data below, the IQ gap should be 0.7 SD for this cohort. Given the correlation found between achievement scores and IQ (0.7), the minimum that the achievement gap should be, given this IQ difference, is .5 SD (0.7 x 0.7). Or conversely, given the achievement gap of maybe 0.2 SD — averaging the Black Caribbean and Black African scores — the most the cognitive gap should be is .3 SD (0.2/0.7). Why the disagreement in data? This, I suggest, is a question which warrants further investigation. Something is amiss in the UK.
There is a second puzzle here, too. The data shows that contemporaneous British Blacks age 30 and up are deficit in cognitive ability around 0.8 standardized units relative to Whites. The data implies that this magnitude of differences has been there for 40 years since testing began. If, for this last generation, the gap is greatly reduced, when did the narrowing begin? What is the magnitude of the gap for contemporaneous British Blacks age 20 and up?