My mate's mum is a single parent, and has been since at least my mate was 7 (she's now 17 - and I've only known her since 7, so maybe before lol). She has 2 children, they don't get anything from the father, they never see him or anything. The woman has no boyfriend or anything, and she does a brilliant job, and has done since I can remember. Infact, they're more well brought up than some of the children around here with 2 parents around (married or not).
So I also think it depends on the situation, as I know it's not always like that. Also think it depends on whether they have support around them - like family and friends ( I think ) And on how lazy they are (If they have a job or not).