These reports examined 5 types of phobias—agoraphobia (AgP), social phobia (SoP), animal phobia (AnP), situational phobia (SiP), and blood/injury phobia (BiP)—and reached the following conclusions. First, all phobia subtypes are moderately familial. Second, familial aggregation of AgP, SoP, and AnP is due to genetic factors. For SiP and BiP, results based on our first assessment15,16 suggested that twin resemblance might result from familial-environmental factors. However, when our analyses included a second assessment interview,17 the added power obtained through control of measurement error suggested that the results for these phobia subtypes were similar to those for the others (ie, the familial transmission due solely to genetic factors). Third, high levels of comorbidity were seen between the phobia subtypes.15 Contrary to results from family studies,4 the best-fitting multivariate model suggested common genetic and individual-specific environmental factors that influenced risk for all phobia subtypes, as well as genetic and environmental factors unique to each subtype. However, these analyses did not include BiP, which was assessed at a later wave. Blood/injury phobia differs from typical phobias,18 where exposure to phobic stimuli (eg, snakes, heights, and public speaking) usually produces increased sympathetic activity (eg, tachycardia, increased blood pressure, sweating, and flushing). By contrast, in individuals with BiP, exposure to phobic stimuli (eg, needles and blood) usually increases parasympathetic activity (eg, bradycardia, hypotension, pallor, and fainting). Given these differences, would the genetic and environmental risk factors for BiP be distinct from those for the other phobia subtypes?