They are subtle, but they are there in the book. He wasn't really old enough for his behavior to be 'diagnosed', but if you've lived with someone (or multiple people) who has gone off on an unexpected "adventure," and they survived it and were later diagnosed with a significant mental illness (like schizophrenia or bipolar, for e.g.), there are just certain signs you start to see that cross the line from being eccentric (or "outside the box") to potentially caused by an undiagnosed mental health issue. People can live with undiagnosed schizophrenia for a long time (my brother in law was in his 30's when he finally reached his breaking point)... although my BIL was always "different," there was absolutely no indication whatsoever that he had anything diagnosable - at least not until he was diagnosed and we started looking backward at all the subtle clues. Since my BIL's "adventure," there have been other members of his extended family (cousins) who have had similar breaks with reality... one disappeared completely for 3 years, but has now made contact with his family (though he hasn't come home yet). I can't remember the source for this, but remember hearing more than once that many of America's homeless are people who are battling mental health issues, and being homeless has nothing to do with being lazy or not wanting to work, but simply being unable to conform to society's expectations. There is nothing wrong with that - everyone is entitled to live the life they choose to live. My only point is that "Into the Wild" is not about a wilderness adventure at all, it is about someone who chose to walk away from society and who just happened to be drawn to a large wilderness area instead of a large urban area. I don't recall getting any feeling whatsoever that Chris intended to return to society at any point in his life.

MNS