Hi Ladies,

My sister's high school biology textbook claims that everyone's hair is genetically programmed to grow to a certain length.  (Could that explain why I can't get past tailbone???)  Anyway, if you CUT it, from the ends,  how do the roots know to grow back to its terminal length?

I am soooo very clueless here!  Any long-haired scientists or biologists out there?

As far as I know, terminal length depends on 1)the growth rate of hair 2)how long individual hairs live and 3) shedding rate. Of course, a lot depends also on how one cares for her hair, how much she cuts etc. I've heard people often claim, that in most people this results roughly to classic length hair. However, many people can grow much longer than that, and only very rarely the hair remains shorter. Many more people claim though, that their hair "just doesn't grow". Usually when one takes a closer look it's evident that the problem isn't with terminal length but with treatment of hair- to reach very long lengths hair requires good, gentle care. It is normal that when hair grows very long, it has resting phases that can last very long -over a year- and during which the hair doesn't grow. After that, it can resume growth. Your hair has probably (not certainly!) reached terminal length when you have recorded no growth for at least one year and half AND your ends have a thinning, tapered look "fairytale ends". Note, that observing just one of these signs isn't a sign of terminal length: many people have tapered ends, but their hair still grows and likewise there are people whose hair is in resting phase: doesn't grow, but ends look still thick.

To your question...not all of your hairs are the same length, ever. You have at all times hairs in all stages of growth. When you cut, you can reach only so many of them, the longest hairs. Though you may shed those hairs sooner than the hairs that you didn't get at when you cut, there are always enough "intact" hairs that can grow to their maximum length if you don't keep cutting. Which you see as hair growth.

This is just how I reason it, not scientific fact.

I had always wanted to know this, but always forgot to ask, so a big thank you to Christiana!!