I believe it is genetic rather than personality Blan.
But it is not as straight forward as that. Life experiences, nurture etc will dictate how we use what we have.
You can have courage without having the 'fight' response...it is not that simple, remember we are talking about an immediate response to a dangerous situation.
You can go into all levels of this sooo deeply, your example of WW1 soldiers vrs today's. Their way of life, their understanding of things, what they had to lose would have been so different from today's soldiers, that could dictate how they behave...and that could sw