Hemophobia causes

According to scientific sources (Testing a Genetic Structure of Blood-Injury-Injection Fears)

Scientists made statistical research with 1800+ participants who afraid blood.
And result is: genetic factor that gives rise to fainting is 64% of the variance. Unique environmental factors are also important.

Conclusion citation: "In conclusion, the data indicate that like other specific
phobias, fear of blood, injury, and injections are
substantially caused by environmental factors that are unique to the individual.

However, this phobia is unique in that some sufferers also faint in the present
of their phobic object. The present data found that such
fainting was caused by genetic factors that were shared
with fainting in other contexts and that this same genetic
factor gave rise to blood fears."

See article if you what to read more about blood phobia causes - Testing a Genetic Structure of Blood-Injury-Injection Fears

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