Some of us are born to be a pessimistic, says study. The chemical NPY in the brain affects the way in which we see the world.
Researchers at University of Michigan discovered that levels of a molecule called neuropeptide Y (NPY) directly relates to whether we have a "glass half empty" or "glass half full" attitude towards life.
Those with lower levels of the substance are much more negative and find it more difficult to deal with stressful situations. They are also more susceptible to depression.
The research team believes the amount of NPY in the brain is genetically programmed and hopes the discovery could lead to early diagnosis of and prevention of psychiatric illness.
Using functional magnetic resonance imaging (fMRI), the scientists scanned the brain activity of a number of volunteers as they viewed neutral words (such as "material") negatively charged words (like "murderer"), and positively charge words (like "hopeful").
In response to negative words, subjects with low levels of NPY showed strong activation in the prefrontal cortex, which is involved with processing emotion, while subjects with high NPY demonstrated a much smaller response.
In a second test, healthy subjects reported their emotional experiences during a stressful challenge.
Saline solution was injected into the jaw muscle, which produces moderate pain for 20 minutes, but no lasting harm.
The level of pain was adjusted for each person until it was, for them, a four on a scale of one to 10.
These subjects rated the positivity or negativity of their feelings both before and after the pain challenge.
Those with low NPY were more negative both before and after the pain - meaning they were more emotionally affected while anticipating the pain and while reflecting on their experience immediately afterward.
Lastly, scientists compared NPY levels with major depressive disorders to see if there was an association between the condition and low expression of NPY.
Subjects with low-expression of NPY were more likely to suffer depression, it was found.
"We hope they can guide us toward assessing an individual's risk for developing depression and anxiety," the Telegraph quoted Dr Brian Mickey, a psychiatrist and lead author, as saying.
The findings are published in the Archives of General Psychiatry.