Fewer than 5% of adult males retain the straight anterior hairline seen in young boys. Some men develop noticeable bitemporal recession and this may precede hair loss elsewhere on the scalp by many years. Hair loss on the crown starts around the whorl (at the back of the head), and spreads outwards in all directions to produce a circular baldness.
Each follicular unit has a primary hair that is present at or shortly after birth. This is why hair in babies is fine and light and downy, but becomes thick and bushy by school time.
When androgenetic alopecia first starts it preferentially shrinks the secondary hairs, so that the follicular units on the affected scalp only produce one terminal hair rather than a tuft of hairs.
It is only when the primary hair - the last remaining fibre form the tuft disappears that bald scalp emerges.
All men lose hair progressively as they grow older. For others it is obvious, and when hair loss is severe or occurs at a young age can be very distressing.
The hallmark of male pattern balding is that hair loss progresses in a distinctive and highly reproducible pattern.
While this sounds straightforward, in fact there are three main areas of scalp that lose hair preferentially, and the relative loss in each of these areas produces variations in the pattern of progression of the hair loss. Hair loss in the temple starts at the anterior hairline and moves backwards.
It occurs to some degree in all boys as they transition from adolescence to manhood.
This pretty much rules out a diffusible chemical and suggests the process of hair miniaturization that shrinks the hairs to fluff is already programmed into the hair follicle DNA.
In addition to the obvious pattern of hair loss that we are all accustomed to seeing, we discovered a second, invisible pattern of hair loss that produces the hair thinning that precedes the balding.