Some of the male characters have more hair in the beginning of the series, i.e. the shaggy look... like Charlie from Numb3rs, Dr. Chase from House, or Chuck/Morgan/Casey from Chuck.
This might be partially related to the fashion at that time, i.e. mid-2000's? Although that is debatable. Another factor might be character development, as the shaggy look (either straight or curly) conveys a more youthful and perhaps inexperienced vibe?
Or just for the sake of variety. Although I have never seen it go the other way, i.e. the character having more hair as the series progresses. :-P
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