Bite My Nails? (Questions)

I seem to have this horrid habit of chewing on my nails, and I have had it since I can remember. My mom and dad both have this habit, but my two older sisters do not. For a while now I have stopped, but now and again I will be sitting and suddenly realize that I am nibbling on them. I really do not mean to. It just happens. It is a bit of a let down when I accidentally look down and one nail is half chipped off. It takes like a month for it to grow back, which just sucks. My nails are doing better now since I have forced myself not to bite them, but it's super hard!

What I’d like to know is:

1) Do you think it's psychological?

2) Do you know of ways to stop?

3) Do you think it's a turn off when people do bite their nails?