You've also got the risks of people who seemingly live happy, healthy lives suffer an incident in their life which changes them. For example, post natal depression. Who's to say someone won't come back to work following pregnancy and then causing havoc due to the illness. They have a clear medical record and no gaps in their employment. So there is an element of risk both for new joiners as well as long term employees. Take the sad example of a long term AA flight attendant who had a meltdown and was sickeningly turned into a piece of entertainment by the deplorable passengers and the news channels.