I nerdsniped myself with observation this morning that there is no U.S. president whose (last) name began with ‘S’.
I bring you this more detailed investigation. The red bars are data taken from the 2010 U.S. Census, and include all names that occurred at least 100 times; that's the 162253 most common names.
For example, about 9.5% of U.S. people counted in 2010 had names beginning with ‘S’. If U.S. presidents matched this rate, we would expect around 4.3 presidents to have had ‘S’ names. Instead, there are none.
In contrast, there are 4 presidents whose names began with ‘J’. (Jefferson, Jackson, and two Johnsons.) Only about 8–9 million Americans have names beginning with ‘J’. If ‘J’ names occurred at the higher presidential rate we would expect more like 26 million.
The correlation of the two variables is 55%.
Incidentally, there has never been a major-league baseball player whose name began with ‘X’. Dean Xingu was a pseudonym for his real name, Dean Zorn.