I always thought of it as a tolerances thing. The stronger/fitter (for certain definitions of those words), and the smarter (ditto), the more slack you have bui

I always thought of it as a tolerances thing. The stronger/fitter (for certain definitions of those words), and the smarter (ditto), the more slack you have built into the system. On the average space flight, an average person is just fine, but when things go sideways, it's better to have someone with very exceptional abilities. That's also not to say that seeking "perfect" astronauts selects for the most ideal abilities, but that was my thinking about what they were going for.