Some are exaggerated forms of evolved fears that all of us share. For example, humans – like some other apes – are naturally wary of snakes, spiders and rats, which makes good biological sense. These natural fears can then be strengthened or weakened depending on how our parents respond to these triggers.
The same kind of emotional learning happens in other species whose infants watch and learn from their parents’ reactions to the world. The fact that a phobia seems irrational to the person suffering from it does not make it go away. Other phobias arise from life events, such as being chased by a seagull or trapped in a small space, but many have no obvious origin and are difficult to explain. They can often be eased by appropriate treatment.