From what I know the causes of alopecia aretea are either hormones, genetics or stress.
The curious thing about my case is that I started going bald when I was 17, my older brother also started going bald when he was 17, so did my father and his father. Can this be explained through genetics? Do our genes "know" what age we are? And if so are they able to trigger a response at a certain age through the generations or is my case just chance?

Also, if hormones are responsible for hair-loss, why does it only affect the top of the head in most cases? Why not the back, the sides, the beard or other body hair?

Thanks for any answers!