Saw this interesting and illustrative picture on Wikipedia of the top newspaper editors in 1885, and the way their hair parts break down is pretty startling – mainly because I was under the impression that the side hair part was not as prevalent back then. But maybe among the 4th estate, which drives the public opinion and conversation did indeed have the side hair part bias way back then:
Here are the numbers as best i can discern from the photo (click for full resolution)
Left part 13/19 (68%)
Right Part 3/19 (16%)
Center/Bald 3/19 (15%)
A fuller study is needed to examine many old pictures to see how these statistics compare to a more general sampling of the population.