What differentiated those who get PTSD and those who get PTG? Oddly enough, they often share the same characteristics (in other words, you can get both). Women, young adults and minorities both have a higher share of both, as do those who are extraverted, conscientious, agreeable, open to experience. Subjective ratings of the intensity of the trauma, a problem-focused coping style and high stress are related to both as well. Higher intelligence is related to lower levels of both.
Okay, similarities out of the way, we move to differences. PTSD has poor social support, PTG good social support. Negative religious coping (religious discontent and focus on punishment) lead to PTSD and positive religious coping (forgiveness, benevolence) to PTG. The neurotic get PTSD much more often, and optimism to PTG.
What these have in common for both very good and very bad outcome is that the experience is defined as very important, particularly as regards identity. PTG means that beliefs are critically examined and challenged in light of this experience. This important event can then lead to beliefs the self is critically damaged or that through that experience important learning has taken place through which the individual finds redemption.
What seems critical to achieving the latter is to regroup after the shattering of their belief system of this experience and put together a coherent and specific new view of the world, a new understanding in the light of this experience.
Boals, A., Schuettler, D., & Southard-Dobbs, S. (2015). Construing trauma as a double-edged sword: how narrative components of autobiographical memory relate to devastation and growth from trauma. Clinical Perspectives on Autobiographical Memory, 65.