Whizzes at Google & Harvard got together and Culturomics is the result. according to this scintillating Science article on the topic, 4% of the books that have ever been published are waiting to be analysed by you, right now. Here is an example of a search I ran to see how things got explained over the last century.
I searched for "psychological explanation" (red line), "sociological explanation" (blue line) and "evolutionary explanation" (green line). Nature appears to be in the ascendant, just overtaking nurture at the close of the century. Sociological explanations peaked in the 1970's, presumably just as the '68 generation came in from the streets and sat down at their desks. Interesting that psychological explanations took a dip around the 1930s. Anti-semitism, maybe?