Using 47 indicators of socioeconomic development and various sources of cognitive test scores, we constructed a socioeconomic development index (SDI) and a set of national intelligence quotients (IQs) for 197 countries, with the latter excluding geographic imputation. Combining the different data sets reduced the estimated standard error of national intelligence quotients (IQs) from 5.41 to 2.58, and a strong correlation was observed between socioeconomic development and national IQs (r = 0.88). Based on the previous assumption that the benefits of the Flynn effect do not exceed measurement invariance, IQ scores should show some non-negligible variation between countries. Empirical assessments of measurement invariance in different countries show that violations of measurement invariance are rare and usually occur for verbal tests. In most countries, national intelligence quotients (IQs) show a high level of reliability and accuracy, and we encourage their use in the scientific literature.