Genetics
Genetics determine beard thickness and fullness. Basically, if your father and grandparents have thick beards, you probably will too.
Ethnicity
Your ethnicity may also affect your beard growth. Chinese men have less facial hair than Caucasian males, and Mediterranean men have the highest, according to studies.
Age
Your hair may thicken with age. Facial hair development slows as a teenager but accelerates about 30.
Alopecia Areata
In alopecia areata, the immune system destroys its own hair follicles. Hair loss can be significant and occur on all body parts.
Testosterone
Testosterone won't modify your beard's patchiness, but it can affect its growth.
Diet and Exercise
Some men can grow a beard, but it's not healthy. It can appear coarse, spotty, and dry. Diet and exercise may be to blame.
Stress
Stress that makes you want to rip out your hair is different from stress that inhibits your hair from growing. Stress can cause hair loss.