neither, we are neutral at birth and the effects of life change us to think one way or the other.

It depends on if they have a purpose or not. Someone who has no purpose, and relies on others, will be pessimistic. Someone with unbending intent will do what it takes to achieve that goal. That’s an “optimist”.

I don’t think humans are naturally either.

I think humans are naturally optimistic. If we were all pessimistic and depressed I don’t think the human species would have survived very long.

Neither. At a young age, our brains aren’t experienced enough to determine such things.
It’s how we grow up and our surrounding environment that affects how we view life.

It really all depends on our parents. If your parents were pessimistic mostly likely you are a pessimistic person as well.

My generation was an optimistic one; younger people seem to be pessimistic.

Okay, the first two hours of Watchmen was a masterpiece, and the last hour was the worst ending in cinematic history.

…didn’t answer anything but needed to be said.

Humans are both pessimistic and optimistic. Either way they are just emotions that sidetrack us from our main tasks – survival and reproduction.

I think some people lead naturally toward more optimistic ideas, and others pessimistic ones… I don’t think that we’re ‘naturally’ one, and the opposite is an abnormality of sort… I think it’s a spectrum, and we all sorta fall in different levels.

I think emotional state lends itself a lot to our outlook—I tend to lean pessimistic, generally, but if I’m having a really good day, the world seems to be bright, glowing, bursting with life. And if my day is… less than good… then it seems like everything is crashing down on me. You know? It can vary, even within a single person.
I think there are advantages and disadvantages to both—optimistic, and you’re generally happier, but pessimistic you’re more likely to strive for perfection.
...I’m not sure which comes first, whether people are born with a glass half-empty or half-full view, or if the way they go through life shapes the way they see the glass… But I really think that both are natural, and which you see depends on who you’re looking at.

We are both optimistic and pessimistic when we need to be….what makes humans such great survivors is that we are REALISTIC in our predictions of the future.

If we were overly optimistic we wouldn’t avoid those situations that might be logically bad situations to be in. On the other hand, being overly pessimistic would stop us from moving forward.

Our ability to function properly is altered when we are too far in one direction, for example someone with a major depressive disorder, or someone experiencing a manic episode.

Neither. We are naturally cautious, ready to fight or for flight as needed for survival.

Pessimistic in a sense. It always comes down to survival.
Always best to air on this side of caution.

The subconscious mind is more pessimistic by nature because it continuously scans for novelty and danger. Negative emotions trigger fast responses.

The conscious mind is shaped by genes, upbringing and societal norms. It can result in an optimistic or pessimistic attitude. And it can be willingly changed (to a certain degree) using methods as explained for example in

http://en.wikipedia.org/wiki/Positive_psychology#Learned_optimism

http://www.amazon.com/Learned-Optimism-Change-Your-Mind/dp/1400078393/

So it’s our nature that we are capable of turning ourselves into optimists.