The Simpson Paradox

The Simpson Paradox Explained (Civil Right Acts of 1964)

Here is the breakdown of the votes by the House of Representatives by Region and by Party:

How come the Democrats voted "yes" in higher proportions in both regions and the Republicans voted "yes" in higher proportions overall ?????

This is known as the Simpson Paradox. The correlation between the variable "Party" and "Yes" is reversed when the data are aggregated (or when the Regions are analyzed together). 

It happens because the correlation between the column variable (Party) and "Yes" is weaker than the correlation between the row variable (Region) and "Yes". In other words, there is stronger correlation between the "Region" and "Yes" than between the "Party" and "Yes".

More info on Wikipedia