I cant believe how many people are saying 'theres no need for a lock in a family house' etc.. whats the problem with wanting a little privacy sometimes? i've never had a lock on my door, asked when i was about 14 or something and my ma said no (what if theres a fire etc..) and never bothered raising the issue again, but despite asking i don't know how many times for people to at least knock before they barge in, my mum, when she remembered to knock at all, would knock then just walk in anyway, my dad would just walk in alll the time having a go at me for one thing or another / go to bed / turn the light off, and my sister used to just walk in all the time too.. so in the end about 6 months ago i went & bought a bolt, never bothered asking, which has now solved the problem.
I think people need to realise that not everyone's parents have the slightest bit of respect for their privacy!