The Will Rogers phenomenon, also rarely called the Okie paradox, is when moving an observation from one group to another increases the average of both groups. It is named after a joke by the comedian Will Rogers in the 1930s about migration during the Great Depression:[1]
When the Okies left Oklahoma and moved to California, they raised the average intelligence level in both states.
All Rogers was attempting to say was that, in his view, a group of Okies whose mean intelligence is lower than the average of all Okies may be smarter than the average Californian.
The apparent paradox comes from the rise in intelligence of both groups, which makes it seem as though intelligence has been "created." However, the overall population maintains the same average intelligence: moving a person from a low-intelligence group to a high-intelligence group makes the high-intelligence group larger, so the population mean (which is a weighted average of the two groups' intelligence) is unaffected.