My broad-stroke opinion:
Conservatives and Liberals can both be intelligent.
But, the difference in their intelligence occurs upfront, when they are considering the information to define their position.
A conservative is dominated by a "belief structure" where their data-filter is less discerning unless it conforms to their beliefs.
A liberal approaches the data from a discerning perspective and with fewer belief-biases.
This is why the word "truth" has different meanings to them.
@jurban
I have often discussed the 'filter first' phenomena.
I think if you were going to build a binary tree of 'types of people' it would probably be the best first splitpoint.
I think it's more prevalent on the right but certainly not lacking on the left.
Those that do not apply the ideology or belief filter first are regarded as heretics by those that do on both left and right.