Although most discussions today start from the assumption that the secular rise in IQ must be environmental in origin, three reasons warrant giving the genetic phenomenon heterosis a closer look as a potential cause. First, it easily accounts for both the high heritability and low shared environmental effects seen in IQ, findings that are difficult to reconcile with environmental hypotheses. Second, numerous other highly heritable traits, both physical as well as psychological, have also undergone large secular changes in parallel with IQ, which is consistent with the occurrence of broad-based genetic change like heterosis. And third, a heterosis hypothesis for the trend can be tested in several straightforward ways. The paper also provides a hypothetical example, based on data from a real population, of how heterosis can result from demographic changes like those that have taken place throughout the developed world in recent history and shows that under certain conditions, even a small demographic change could cause large genetically based phenotypic changes.