I agree with this graph.
Happiness is harder to attain when you are intelligent, or observant and analytical at least. Intelligent people, simply put, notice too much to be happy.
They are too self-aware. They see their own faults, their problems, the burdens tomorrow and the day after will bring.
They see the behaviors of their surrounding brethren, and are saddened, angered, by senseless debauchery, because they themselves are capable of being better and in turn expect this of their peers.
Intelligent people also place much higher expectations on themselves. Aware of the extent of their capabilities, their personal definition of success, whether consciously realized or not, is greater than that of a less intelligent person. This means they have to work harder, achieve more, to feel the same sense of satisfaction as someone else.
The result? A small percentage of people who are insecure, angry at the people around them without knowing why, and constantly sad - the hollow, empty sadness that indicates something is missing.
There's a reason the overconfident jock and timid nerd stereotypes exist - because, to some extent, they are true. Physical types are far more appreciated because intelligent people are less inclined towards self-celebration, all too aware of their own mental and physical flaws.
So take your pick. Ignorance is bliss, not for the romanticized idea of worldly innocence, but for the vision that allows its user to see into another world, one where deception and self-interest reign beneath the surface of goodwill, silver linings only hide grayer clouds, and the most horrifying spectacle lies within one's self.