* Genetics: This is the most probable cause. Hair growth patterns, including facial hair, are largely determined by genetics. If the men in his family (father, grandfathers, etc.) have sparse or slow-growing facial hair, it's likely Gerard inherited similar traits.
* Age: While Gerard is no longer young, facial hair density and growth can continue to develop into a person's 30s.
* Hormonal Factors: While less likely if it's a long-standing pattern, hormonal imbalances can affect hair growth. However, this would usually be accompanied by other noticeable symptoms.
It's important to remember that the ability to grow a full beard is not an indicator of health or masculinity. It's simply a genetic trait.