How does the body decide where hair should grow and how thick and how long?

I think you’ll fine the Numskulls decide on those things.

In case you are not buying Brian’s charming explanation, genetics has this to offer (although it does not seem to explain your cast situation):

“Hair texture is just one of the many obvious physical differences that exist between ethnic groups. Although hair growth rate, size, shape and texture are unique to every person, we can see trends among groups of people.

While genes are likely to be involved in determining these traits, not much is known yet about the actual ones involved. So what is known about hair type?

Hair follicles are tiny pockets in our scalp out of which our hair grows. As you hinted above, the thickness and texture of our hair depends on the size and shape of these follicles. They help to form and contour our hair as it grows.

Our hair thickness results from a combination of both the size of the follicles themselves and how many of them line our scalp. The size of the follicles determines if the individual hair strands are thick or thin. Large follicles produce thick hairs. Small follicles produce thin hairs. It is that simple!”

Or that. That’s what I meant.

Thanks for the response, Marina. I’m still curious why wearing a cast would change the quality and appearance of the hair though.

I am rather curious about this too. I still can’t find out why, but I did find confirmation that it is a common phenomenon:

“After the cast is removed it is normal for there to be some discomfort in the bones and joints that were immobilized, for the arm or leg to be smaller than the other side, and for the skin to have some changes (dry skin and more hair).”

Would this explain it (from a medical blog)...

“Many people that are required to wear a cast see what you’ve reported. This occurs because the arm hair which sheds frequently as the short hair cycle pushes out old hair (a half to one inch in length) is protected in the cast so that it does not fall out day be day. Most people with normal bathing and showers will get these old hairs to fall out by a simple scrub.”