What is the average IQ worldwide?

IQ tests are built so that the standardisation sample averages 100 with a standard deviation of 15, so the 'average IQ' is 100 by definition rather than by measurement. Raw scores rose about 3 points per decade through the 20th century (the Flynn effect), which is why tests are re-normed every 15 to 20 years. Country-level IQ comparisons are contested because sampling and test translation vary a lot.

Source: Wechsler, WAIS-IV technical manual (2008); Pietschnig & Voracek (2015), Perspectives on Psychological Science