The Boundary concept was developed by Dr. Ernest Hartmann, of Tufts University, and can be a useful way of looking at personality differences and understanding why one person may develop a chronic illness that is distinctly different than a chronic illness that someone else develops.
Boundaries are more than a measure of introversion or extroversion, openness or closed-mindedness, agreeableness or hostility, or any other personality trait. Boundaries are a way to assess the characteristic way a person views her/himself and the way s/he operates in the world based on how that person handles the energy of feelings. To what extent are stimuli let in or kept out? How are a person's feelings processed internally? Boundaries are a fresh and unique way of evaluating how we function.
According to Dr. Hartmann, "There are people who strike us as very solid and well organized; they keep everything in its place. They are well defended. They seem rigid, even armored; we sometimes speak of them as "thick-skinned." Such people, in my view, have very thick boundaries. At the other extreme are people who are especially sensitive, open, or vulnerable. In their minds, things are relatively fluid. Such people have particularly thin boundaries."
- See more at: http://psychcentral.com/cgi-bin/boundary-quiz.cgi#sthash.7BBcCFl2.dpuf