Prof. Groseclose made an interesting statement near the end of the video. He stated that without the liberal bias of the media, most states would resemble Texas in voting patterns. Maybe, but fifty years ago the great majority of Americans, Dems and Republicans, were much closer in their views on practically everything including social and political topics. In those days, the Dems tried to outhawk the Republicans on national security i.e. Kennedy's "missile gap" statements. Abortion and homosexual "rights" issues were not on the horizon. Which meant the parties were much closer in their beliefs and actions. But the media was just as left-leaning as it is today.
So why is there a higher percentage of Americans who are liberal than there was fifty years ago? I think the lib influence of the cultural and academic worlds on America over fifty years time has influenced the leftward tilt as much or more than the media.
There isn’t a large enough number of people attending college to be a significant cause of the growth of liberals. The cultural influences you mention find their way into our conciouses via media like tv shows and pop music.
Do you think it’s a coincidence that support for gay marriage is slowly growing at the same time there are an increasing number of likeable gay characters on primetime tv?