People differ in their sensitivity to similar experiences with some being generally more and some less sensitive. Several theories have been developed over the years to describe such differences in Environmental Sensitivity and a growing number of studies provide empirical evidence that some people are indeed more affected than others by environmental influences. Sensitivity has been associated with temperament traits in children as well as a number of biological factors. In this paper, we provide new experimental evidence that self-reported sensitivity predicts differences in the response to negative as well as positive experiences, after introducing a short sensitivity questionnaire for adults, building on the original Highly Sensitive Person scale (Aron & Aron, 1997), and investigating its psychometric properties as well as associations with the Big Five personality traits. Results across the five individual studies (total N = 1,140) suggest that individual differences in sensitivity to the environment can be assessed with the newly developed measure (HSP-12), validated by two experimental studies according to which the scale predicts heightened reactivity to both negative and positive experiences. Furthermore, sensitivity reflects a continuous spectrum from low to high along which people fall into three distinct sensitivity groups (low, medium, and high). Finally, sensitivity was found to fit a specific personality profile characterised by both high Neuroticism and high Openness to Experiences. We conclude that adults differ substantially in their sensitivity to environmental influences and that such differences can be measured reliably with a short and easily applicable sensitivity questionnaire.