For the first time, empirical evidence allowed to construct the frequency distribution of an index related to the degree of genetic relatedness between the parents of about 0.5 million humans living in the UK. The results show that a large proportion of the population is not the product of parents choosing a mate randomly. Assortative mating leading to offspring, that occurs between genetic related individuals, is very common. High degrees of genetic relatedness, i.e. extreme inbreeding, is rare. The evidence shows that assortative mating is highly prevalent in this large population sample. This novel empirical result suggests that assuming random mating, as widely done in population genetic studies, is not an appropriate approximation to reality.