The simplest reply I can think of is 'ignorance is bliss,' and it's easy to see how that phrase/thought came into being. I don't know if there is a definitive way to prove there's some universal rule or something that demonstrates +intelligence = -happiness, but there are a lot of reasons to see why this would be the case. the more you know, sometimes you find all the more to worry about or otherwise be concerned with. It can also be very difficult to truly  'un-know' what you know, or 'un-learn' what you've learned. Sure, people forget things all the time and that's normal, but if you truly have become aware of something, or feel you've become aware of something, it can be almost impossible, if not actually so.

Keep in mind though that personality type also has a great influence, if not the biggest. Plenty of very intelligent people are continuously very happy, because that's just the kind of personality they have; they keep an excessively positive outlook because....they just do. People can always decide to think positively, but some people just do it almost instinctively. However, I guess it is possible for anyone to reach some kind of limit where they come into contact with some kind of information that just completely changes them, and they become 'awakened,' a word that's used a lot to describe such a sensation, and this could completely change anyone, possibly even irrevocably. A person that was once excessively positive by apparent instinct could become the opposite in such a case, depending on the information acquired and how the person understands/deals/copes with it. Thinking too much about just about anything can naturally cause unhappiness, especially when one attempts to completely dissect every little thing, refuses to accept what they don't or can't understand, or has a tendency to just look at or make things more complicated than they really are or should be.

Another important thing to keep in mind is the question  of 'intelligence' itself; how is it being defined? The problem with the term is that it's meaning isn't actually that clear, especially when the question arises of how to actually measure or determine it. While one may not have the 'intelligence,' as it's defined and displayed, of Stephen Hawking, that does not mean that thus, one is 'not intelligent.'

I think 'intelligence' can certainly block the path to happiness, especially if one is the kind of person who thinks they truly are 'smarter' than everyone else; that they truly are exceptional people, and everyone else 'just doesn't understand.' I'm not saying such a thing never legitimately happens, but the problem is, it also happens plenty of times because some people are really not as 'smart' as they think they are, and refuse to accept their situation any other way. Again, this is often just the result of personality type and I'd also say a state of denial. I think this goes along with what you said, Gnashvar, that "Just because you're smart it doesn't mean you have the knowledge to know what could be wrong or right with your life." [emphasis added]

I think it's also fair to say that sure, plenty of people who are 'not smart' are happy, but many also are not. Like you suggested, "most suffer as much  as everyone else." At least, I know of no way to accurately determine that one group 'suffers' more than the other, if such a method even exists.

As far as the Garden of Eden goes, I believe the main point of that story is actually about disobedience to God, more than anything else, as God commanded them not to eat the fruit, and they did anyway. This also helped separate humanity from God, and created a void in all people that nothing else could fill; that we all have a need for God within us, and Adam and Eve were left without the near-presence of God, and that did lead to an unhappiness. Of course, you can still interpret the story however you want, too. :P

Great reply.

So we could actually define that it is knowledge that might be  causing unhappiness not so much intelligence.   It would seem that intelligence is the culprit because most of the time people with higher than average intelligence are the ones that seek knowledge.    

Personality as you mentioned seems to be very important probably more so than the knowledge.  But could it be that the same knowledge shapes your personality?

One of the things that made wonder about the Garden of Eden is the part after eating the fruit where they realize that they were naked.   When they became aware they had no clothes.  Self consciousness.   So yes it means that they disobeyed God but it also created unhappiness when they realized they had desires and needs.   I wonder if another animal worries  or desires.

Animals don't have 'worries' or 'desires,' exactly; they act on instinct and don't have the ability to understand the concepts. This is soley part of the human condition. :/

Well, the thing is, one can certainly increase their 'intelligence' in a number of ways by seeking and obtaining knowledge, yes; and people of every level of 'intelligence' seek knowledge all the time. The issue would be how is 'intelligence' being defined. It's not necessarily true that "most of the time people with higher than average intelligence are the ones that seek knowledge." In fact, I'd say it's not true, because people who don't have 'higher than average intelligence' seek knowledge in a variety of ways all the time. I don't think it's entirely fair or accurate to call intelligence any kind of 'culprit' though; sometimes a person is the 'culprit' themselves by refusing to seek out new information and knowledge. Hey, some people prefer to remain ignorant of something, are too lazy to educate themselves, and/or simply don't care.  How many times have you seen it happen, probably every day even? That's another problem in itself, but it's like the proverb says; 'you can lead a jackass to water, but you can't make him drink.' Worse yet, some people think they already know something important, or more importantly, 'the truth,' but well the real truth is, they need more education on whatever it is they think they know. This goes along with what I was saying before about denial in a lot of people.

This also reminds me of a Calvin and Hobbes strip. Calvin was complaining about school, as usual, and he didn't like the fact that he had to go. Hobbes asks, 'Is it a right to remain ignorant?' Calvin replies, 'I don't know, but I refuse to find out!' >D

I think the realization of desires and needs goes along with what I previously mentioned about the need for God, and the separation from God within one's self that was created after the fruit was eaten. Eden was the Earthly Paradise where there wasn't really such a thing as 'needs' because they were...well, unnecessary! Many, if not most, Christians believe that this is what Heaven would be like, as well; a place where we return to not 'needing' anything anymore, and the separation from God has ended. I realize that a lot of people who don't believe in Christianity, and especially those who despise it, would find all sorts of other negative meanings into the same section; this is just how I understand it. That's neither here nor there though. :P

I disagree that ignorance is bliss because I usually find my greatest pleasures in more intellectual activities. And those I enjoy that are not more intellectual are not any less for having intellect.