De novo mutations are important both as sources of diversity in evolution and for their immediate impact on diseases. Scientists at deCODE genetics and their colleagues have used whole-genome sequencing data from 78 Icelandic parent–offspring trios to study mutation rates in humans at the genome-wide level. They find that diversity in the mutation rate of single nucleotide polymorphisms is dominated by the age of the father at the time a child is conceived. For each year increase in the father's age at conception, the number of mutations increases by about two, and once the effects of random variation are accounted for the father's age is estimated to explain almost all of the remaining variation in the de novo mutation counts. Furthermore, the results show that demographic transitions that affect the age at which males reproduce can have a considerable effect on the rate of mutations, and consequently on the risk of diseases such as schizophrenia and autism.