Almost anything can become the cause of a phobia, but there are three broad categories. Most frequent are phobias of certain physical situations such as flying, heights, driving on motorways, being alone in public places away from the safety of home, and being trapped in small spaces such as lifts or buses or trains. The second category involves social situations such as having to eat, drink, speak, write, or perform in front of other people. The third category consists of phobias that are linked to particular animals, notably spiders (or other insects), snakes, birds and dogs.

The fact that certain phobias (e.g. flying, being trapped, spiders) occur much more frequently than others (e.g. mowing the lawn, or lambs) suggests that genetic factors may be involved which predispose people to learn to fear certain things that actually were dangerous to the survival of our human ancestors. However the fact that almost anything can become a phobia, and that most adult phobias are not present at birth (or even in infancy) but can usually be traced back to childhood or later experiences, points to the importance of learning in the development of phobias. One way in which they can be learned is by associating a certain situation (e.g. sitting in a dentist's chair) with great pain (e.g. the drill hitting a nerve!). Another is by observing the fear reactions of other people, particularly those who are significant to us in childhood such as parents, older siblings or friends.