Why is my hair thinner on one side?

I am 46, and for the last 10 plus maybe 20 years I have noticed that the entire right side of my head has much thinner hair than the left. In fact, I would say the left side is twice as thick. As I get older, it is obviously more noticeable. When my hair gets longer it is really bad.....I am just wondering if I am the only person to ever experience this. If you take a line and basically divide my head in to 2 sides, the right has half as much hair. Front front to back and side.