Developing countries achieve surprisingly weak results in international cognitive competence studies. The results are about one to two standard deviations below the average norm-values obtained in Western countries. The results are so low that they are sometimes difficult to believe. For example, in the World Bank collection, Nigeria scored 262 student assessment points (SASQ, about two and a half standard deviations below the norm 500, equivalent in the IQ-scale to 64 points); in the Lim et al. collection, Yemen scored 336 SASQ (equivalent to IQ 75). These results have triggered opposition, factual-scientific criticism, but also ethical debates and political-ideological objections. We crosscheck the values here by comparing different sources of information, statistical analysis, and on-site reports. Results of the different test paradigms seem to be similar for country groups. However, there are sometimes major differences for individual countries. The same is true for comparing different test collections: results of grouped countries are similar, but single countries differ. Using education, GDP per capita and politics to predict test scores shows somewhat higher results for the Global South (5 IQ points), especially for Latin America (rising from 78 to 86 points in an IQ metric). Finally, observational studies of schools and every day life in several countries of the Global South point to problems in the scope and quality of instruction as well as of thinking. To improve outcomes and competences, it is recommended to expand education (e.g., kindergarten, extension of schooling) and better train teachers. Where there are large discrepancies between predictions and test results, the potential seems to be large.