Contemporary theories of fears (and phobias) suggest two kinds of etiologic factors; those common to all fears (factors influencing fear-proneness) and factors specific to particular fears (e.g. specific learning experiences). There also may be etiologic factors of intermediate specificity; factors common to some but not all fears. The present article describes this hierarchic model of fears, which proposes that fears (and phobias) arise from a hierarchy of causal factors, ranging from specific to general. The model is supported by factor-analytic studies of fear inventories and behavioural-genetic studies of twins. However, further research is needed to clarify the number of levels, and to identify environmental, psychobiological, and genetic factors at each level of the causal hierarchy. The hierarchic model of fears represents a call for research. It encourages researchers to take a broad view of etiology by considering factors at various levels of causal specificity.