I have been growing out my hair for 15 months, and its about 8 inches long. It just touches my shoulders, and I've noticed that hair on my left side is slightly thicker and longer than my right side. It's not by very much - only half an inch or so. It's interesting, because I also notice that my moustache grows slightly longer on my left side too. Does anyone else have a similar experience?
For me, the differences between left and right seem to reflect the pattern of my hair growth, rather than a specific difference between the amounts of hair on each side. My hair and whiskers grow in a strongly clock-wise direction (when viewed from above), which produces noticable differences in the way they look and behave when they get longer.
For example, the hair on the right side of my head brushes back easily but on the left side it stands up more because brushing it back goes against the direction of growth. Similarly the hair on my neck easily sweeps back around my neck on the right but curls forward toward the back of my ear on the left.
I particularly notice the grain after several days of not shaving. The whiskers on my neck lay almost horizontally right to left.
I've also suspect that some differences between left and right may have to do with eyesight. For example, for the life of me I cannot trim my sideburns perfectly evenly -- one is always a little longer and thicker than the other, which lends the appearance of disproportionate growth. Trimming the right side is just easier for me, hence the difference.
Jim