Most royal candidate theory

The most royal candidate theory of Harold Brooks-Baker proposed that the winning candidate in United States presidential elections had always been the candidate with the greatest percentage of "royal blood" in his pedigree, and this pattern could be used to predict the outcome of upcoming elections. Brooks-Baker promoted it during several election cycles, ending with the 2004 presidential election, the last before his death.

Source: Wikipedia — Most royal candidate theory (CC BY-SA 4.0)