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A sample of 175 same sex dizygotic pairs and 144 monozygotic twin pairs aged between 8:00 and 18:00 completed the Fear Survey Schedule for Children--Revised. The heritabilities were significant for Fear of the Unknown (h2 = 0.46, p less than 0.001), Fear of Injury and Small Animals (h2 = 0.46, p less than 0.001), Fear of Danger (h2 = 0.34, p less than 0.001) and for Total Fear Score (h2 = 0.29, p less than 0.001). Multiple regression was used to estimate the heritability of extreme fearfulness (h2g). For each of the fear factors the values of h2g were of similar magnitude to those of h2 suggesting that there is no evidence of greater genetic influences at more extreme levels of fearfulness.
Stevenson et al. (Tue,) studied this question.