A significant set of studies had shown that the strongest correlate of liberal was being open in the Big 5 personality inventory, and conversely so for conservatives. Openness is typically defined as open to experience, but is alternatively describes as intellect. There are also obvious correlations with intelligence/education with higher levels of IQ and education correlating with greater degrees of openness. A quick look at the job classifications would indicate that, generally, smarter people with abstract jobs are liberals, while less intelligent people with concrete jobs are conservative, with the biggest exceptions being the physical sciences. Obviously, the biggest difference between the two groups, of liberal versus conservative, was whether the job was abstract or concrete in nature.
musings on life and liberty