Backwoodsman: Your friend was hardly being original. I knew I had heard the comment before, but wouldn't have been able to pin it down myself. Google helps:

""Raised the IQ of both" refers to witty remarks, notably by Will Rogers about Okies moving to California and Robert Muldoon about Kiwis moving to Australia, implying that people leaving a less-developed area for a more advanced one (or vice-versa) boosts the average intelligence of both regions by shifting populations, highlighting migration patterns and regional disparities in opportunities. It's often cited as a humorous way to describe population migration's statistical effect on average intelligence, sometimes called the "Will Rogers Phenomenon" in medicine for similar statistical paradoxes."