We aren't born with phobias. Babies don't seem to get frightened by things like insects, confined spaces or any of the things we associate with phobia. These fears usually do develop early though, mainly after the age of four. It's thought that the best known phobias probably develop simply as a result of being frightened by something when we are old enough to be aware of it but not old enough to understand and deal with it. For example, infants aren't worried by insects or spiders, but a child of three or four who suddenly finds a spider running over his or her bed may get a real shock and develop a lifelong fear of them.

Other phobias are harder to trace. There is a lot of speculation on how you develop fears like social phobia, which often starts later in life. It is though some people may be genetically more disposed to become phobic - not that you can 'iherit' a specific fear, but you could be more likely to develop them given certain conditions.