To a certain extent, people self-select for these different types of occupations. For instance, we don't have any screening criteria for submariners. The people who end up in that field have self-selected to be comfortable in that close, confined environment. The Americans do a screening process.
I think in our military, most people self-select to the different fields, knowing that they are going to be away from home and knowing that they're going to be employed in that fashion.