Here's a safety poster from World War II. It enumerates desireable qualities of the aviator: sound body, clear mind, steady nerve, and most of all, self control. I saw it and thought, "well okay, that's one way to put it," but I had no idea what the poster was really driving at.
Now, it's clearly important for an aviator or aviatrix to have self control such that he or she remains calm in emergencies, resists the temptation to race his colleagues while in command of passenger jets (you know who you are), and is disciplined enough to cancel or divert a flight as necessary, despite personal or company pressures. Apparently that wasn't the problem the poster was designed to combat. They're talking about the self control required to avoid certain medical conditions. Another poster provides more context.
I can safely state that while I've seen flights cancelled for weather, mechanical malfunction, availability of fuel or deicing solution, lack of business, continent-wide terrorist-inspired air travel system shutdown (yes, then), pilot illness, or pilot forgot to renew his medical, I've never seen a mission cancelled for this reason.
I suppose this is an issue in every war. It figures I'd find this the day after I registered my blog on a site that specifed "no sexual content."