Further evidence of a connection between brain physiology and psychological states has recently been uncovered in the form of a correlation between electroencephalograph patterns and characteristic moods  A study showed that participants who suffered form clinical depression exhibited less left frontal lobe activity than right, while, conversely, characteristically good-natured participants exhibited greater left lobe activity
Thus one' s general disposition is a result of the activity of one' s frontal lobe
Frontal lobe activity is not subject to variation the way general disposition is.