Although doctors have confirmed this as a myth, i am still a strong believer that it does. Doctors believe that hairs only appear thicker because the end of the hairs have not blunted out yet. However, my brother has shaved his head several times and grew it out long for a while. His hair was very thick and healthy, even at its longest (up to mid back) as opposed to mine which is very thin and thinning at 22 years old =( (i'm a girl). When we compared a strand of hair, his was twice as thick as one of mine! I've been seriously considering shaving my locks but i can't bring myself to do it!
Some people will argue that if this was the case bald men would shave their heads all the time in order to grow more hair, but bald men (and women in some cases) are bald because the follicles in their head have died and hair growth cannot be promoted in those areas. Thicker hair means thicker strands not necessarily more hair so if a bald man was to shave the remainder of his hair i believe the strands would grow back thicker.
People will aslo argue that me and my brother have different type of hair but our mother confirms that the thickness and volume was the same if not very similar as we were growing up. Either way, although my theory isn't 100 percent accurate i'm a firm believer. Doesn't hurt to try!