[cicatriz]

Not that passion can't reinforce grit, but I tend to think that it's actually grit that causes passion, as Cal Newport often argues: http://calnewport.com/blog/2010/01/23/beyond-passion-the-sci...

So, indeed, what increases grit?

Sayemm's quote about prodigies seems to imply that there is something innate causing a singleminded obsession. Maybe in some cases, but there are cases like Mozart and the Polgar sisters (http://en.wikipedia.org/wiki/Judit_Polg%C3%A1r#Early_life), who were basically raised into the target of their grit and passion.

I think one of the answers, as in the forth point, is environment. Take programming. In my case, I got started at a young age because I had a computer and found a programming book one time when I was bored. From there it was extremely easy to surround myself, via the internet, with people who are gritty about programming, to read books and blogs all the time, and to have projects all around me.

[adamsmith]

The relationship between boredom and grit, or maybe an intermediate variable like finding what you love, is an interesting one to ponder.