A study by Aron & Aron (1997) established that some people are highly sensitive because they have a high sensitivity in sensory-processing, which means that they become easily aroused and overwhelmed by external stimuli.
Therefore, the sensitivity is closely connected to arousability, because the nervous system becomes easily aroused. The trait highly sensitive is partially independent from the trait introversion, which is an important aspect of the research. Introverted people are often mistaken for being highly sensitive and vice versa.
In their research, they found that the majority individuals were very similar to non-sensitive individuals except for their sensitivity of course. On the basis of their research, Aron & Aron have made a 27-item Highly Sensitive Person Scale that serves the purpose of differentiating introverts from highly sensitive individuals. Indeed, there is an overlap between the two traits, but introversion does not account for all of the aspects of high sensitivity.
Emotionality, on the other hand, is substantially correlated with high sensitivity. The fact that highly sensitive people react emotionally to overwhelming stimuli is not surprising because they become more aroused than non-sensitive people due to their increased awareness (sensitivity), and this arousal might foster emotions such as fear and sadness etc.
A following study by Benham (2006) used the Highly Sensitive Person Scale to identity highly sensitive people from non-sensitive people. Benham sought to understand the relationship between high sensitivity, stress and physical symptoms.
He found a positive correlation between perceived levels of stress and symptoms of ill-health. Interestingly, Benham also found that the trait highly sensitive is a stronger predictor of ill health than perceived stress.
The reason why high sensitivity is associated with health issues is that the sensitivity heightens the general physiological arousal, which can lead to a chronic stress condition that, in turn, affects health. With respect to the chronic stress condition, another study also found that high sensitivity is linked to an increase in perceived stress (Gerstenberg, 2012).
However, another reason why these highly sensitive individuals were found to suffer more frequently from ill health could be due to the fact that they are more aware of their physiological sensations and thus feel more ill than people with low awareness.