Do people's hair stand up when they shock? Why?
The small hairs on the back of the neck (called hackles) and on the arms
do sometimes, but not head hair. I don't know why, but it happens in other
mammals, like dogs and cats, with much greater effect because they have
more hair. Perhaps it provided at some point some advantage to our
ancestors in escaping a threat or at least communicating our defensive
state of mind -- note how effectively a dog's hostility is conveyed by its
fur standing up! Now it seems just vestigial like the appendix.
If you want your head hair to stand up go to the van de Graaf generator
exhibit at your science museum, or pull a really furry acrylic sweater over
your head on a dry winter day.
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Update: June 2012