Phobias affect about 10 per cent of the general population at some point in their lives, with women affected twice as commonly as men.
Phobias commonly involve objects and situations that were realistic dangers for our distant ancestors: poisonous or vicious animals and invitations to injury. As a result, many people are terrified of things that no longer pose a contemporary threat.
Ancestral fears are learned with remarkable ease. One study found that young rhesus monkeys acquired a fear of snakes when they viewed a film of older monkeys acting terrified in the presence of a snake, but did not come to fear flowers when they viewed monkeys going ape in the presence of a blossom.
Fears related to things that were threats to our forebears are more easily acquired than others.
Although many common phobias are of this ancient or "prepared" kind, the spectrum of human fears is astonishingly broad. The clinical literature records phobias of rubber bands, dolls, clowns, balloons, onions, being laughed at, dictation, sneezing, swings, chocolate and the wicked, beady eyes of potatoes.
Unusual fears are particularly common among people with autism, who have been known to dread hair dryers, egg-beaters, toilets, black television screens, hairs in the bathtub and facial moles.
It is hard to see the evolutionary threat posed by these innocuous things.
Given that many modern phobias make little logical sense, it is interesting to explore how they emerge.
There are three main identified ways that phobias come about: a terrifying personal encounter, witnessing another person's fright, and receiving threatening information.
A person might acquire a spider phobia after a close encounter in the shower, after seeing a sibling run screaming from one or after being told that spider bites make you turn purple and die.
Only a small minority of people will develop phobias after common experiences such as these. Those who had inhibited temperaments in childhood and neurotic personalities in adulthood are more vulnerable, and this vulnerability has a substantial genetic component. A study that followed a sample of young women over a 17-month period found that those who developed phobias tended to have more pre-existing psychological problems, poorer coping skills and a more pessimistic mindset than their peers.
Let's consider one odd but surprisingly common aversion, the fear of frogs.
One published case documented a woman who developed ranidaphobia, as it is known, after running over a knot of frogs with a lawn mower. Paralysed by fear and tormented by amphibian dreams, she was persecuted every evening by an accusing chorus of survivors on a nearby riverbank.
In another case, a Ghanaian schoolboy developed his phobia when he stepped on a frog while touching itchy leaves. After his brother told him that frog urine could cause itching and a painful death, the boy became paralysed with the fear that frogs were hiding in his bed.
What gives these puny creatures - with big eyes and scrawny, hairless bodies - their power to inspire fear and trembling? They pose no realistic threat to life: phobic individuals understand that in an encounter with a frog they are unlikely to be the one to croak.
The fear of frogs is viscerally unreasonable. To many people it reflects the frog's slimy, skin-crawling ickyness. To others, it is the creature's propensity for sudden movement, a trait it shares with another tiny source of terror, the mouse.
Luckily for phobia sufferers, treatment is generally quick and effective. Cognitive-behaviour therapists have an assortment of techniques for confronting fears and challenging the avoidance and thinking biases that sustain them.
Usually these methods involve progressive exposure to the feared object or situation up the steps of a "fear hierarchy", from relatively non-threatening encounters to the most terrifying.
These "behavioural experiments" are often supplemented by relaxation techniques, modelling of exposure by the therapist and correction of catastrophic thoughts.
In another case of ranidaphobia, a young nursing student fainted in a biology class when her laboratory partner severed a frog's spinal cord ("pithing"). A course of therapy was commenced in which she repeatedly viewed a videotape of the operation and practised relaxation techniques.
Such was the success of the treatment that immediately afterwards she was able to deliver electric shocks to one frog and pith another, remaining calm even when one frog hopped loose, bleeding profusely from its injuries.
By facing what we dread, under the guidance of a psychologist, we can find freedom from irrational fear.
The Conversation
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Nick Haslam is professor of social and clinical psychology at the University of Melbourne.
