[citation needed] A number of studies have cast doubt, though, on whether accident-proneness actually exists as a "distinct, persistent and independently verifiable" physiological or psychological syndrome.
Factors which have been considered as associated with accident-proneness have included absent-mindedness, clumsiness, carelessness, impulsivity, predisposition to risk-taking, and unconscious desires to create accidents as a way of achieving secondary gains.
[4][5] In The Handbook of the Emotions (1993), psychologist Arne Öhman studied pairing an unconditioned stimulus with evolutionarily-relevant fear-response neutral stimuli (snakes and spiders) versus evolutionarily-irrelevant fear-response neutral stimuli (mushrooms, flowers, physical representation of polyhedra, firearms, and electrical outlets) on human subjects and found that ophidiophobia and arachnophobia required only one pairing to develop a conditioned response while mycophobia, anthophobia, phobias of physical representations of polyhedra, firearms, and electrical outlets required multiple pairings and went extinct without continued conditioning while the conditioned ophidiophobia and arachnophobia were permanent.
[7] Psychologist Paul Ekman cites the following anecdote recounted by Charles Darwin in The Expression of the Emotions in Man and Animals (1872) in connection with Öhman's research: I put my face close to the thick glass-plate in front of a puff-adder in the Zoological Gardens, with the firm determination of not starting back if the snake struck at me; but, as soon as the blow was struck, my resolution went for nothing, and I jumped a yard or two backwards with astonishing rapidity.
[10] Psychiatrists Isaac Marks and Randolph M. Nesse and evolutionary biologist George C. Williams have noted that people with systematically deficient responses to various adaptive phobias (e.g. basophobia, ophidiophobia, arachnophobia) are more temperamentally careless and more likely to receive unintentional injuries that are potentially fatal and have proposed that such deficient phobia should be classified as "hypophobia" due to its selfish genetic consequences.
[list 1] Nesse notes that while conditioned fear responses to evolutionarily novel dangerous objects such as electrical outlets is possible, the conditioning is slower because such cues have no prewired connection to fear, noting further that despite the emphasis of the risks of speeding and drunk driving in driver's education, it alone does not provide reliable protection against traffic collisions and that nearly one-quarter of all deaths in 2014 of people aged 15 to 24 in the United States were in traffic collisions.
[20] In November 1999, Biological Psychiatry published a literature review by psychiatrists Joseph Biederman and Thomas Spencer on the pathophysiology of ADHD that found the average heritability estimate of ADHD from twin studies to be 0.8,[21] while a subsequent family, twin, and adoption studies literature review published in Molecular Psychiatry in April 2019 by psychologists Stephen Faraone and Henrik Larsson that found an average heritability estimate of 0.74.