There are many ways of knowing the means objectively. In countries with mandatory military service and where IQ tests are administered in connection wit that se

There are many ways of knowing the means objectively. In countries with mandatory military service and where IQ tests are administered in connection wit that service there is reliable data. Another source is the number of clinical cases of mild mental retardation. This will mostly depend on how large the population subgroups with lowest IQ are, but if they are approximately fixed and there's no explosion in assortative mating it would be possible to calculate the true mean from that.