"Existential depression" is not a euphemism, it has a specific meaning. While, I do not have the primary references to back up my point [so I guess you may take me to task for that] I too am very much immersed in the gifted field. There is most definitely a consensus among the people who deal with gifted children, especially their emotional needs, that existential depression hits earlier and more often in these kids than in the general population of children. I do not know about giftedness in general, including adult giftedness.