Background. Familial factors, which are partly genetic, influence risk for phobias. Prior family and
twin studies, however, were based on a single lifetime assessment, which may be only moderately
Methods. We obtained, 8 years apart, two assessments of lifetime history of five unreasonable fears
and phobias (agoraphobia and social, situational, animal and blood-injury phobia) from face-to-face and telephone interviews from 1708 individual female twins from a population-based registry.
We also obtained, 1 month apart, test–retest reliability on 192 twins. We fitted, using the program
Mx, a measurement model that estimates the role of genetic and environmental risk factors
correcting for measurement error.
Results. Short-term reliability of the five phobias was modest (mean κ=0·46), but higher than long-term stability (mean κ=0·30). Unreliability occurred both for subject recall of unreasonable fears
and for interviewer assessment of which fears constituted phobias. Examining fears and phobias
together, in a multiple threshold model, results suggested that twin resemblance was due solely to
genetic factors, with estimated total heritabilities, corrected for unreliability, of: any 43%,
agoraphobia 67%, animal 47%, blood/injury 59%, situational 46% and social 51%. With the
exception of animal phobia, similar results were obtained analysing phobias alone.
Conclusions. Lifetime histories of unreasonable fears and phobias assessed at personal interview
have substantial unreliability. Correcting for unreliability, the liability to fears and their associated
phobias is moderately heritable. Individual-specific environmental experiences play an important
role in the development of phobias, while familial–environmental factors appear to be of little