In general, gynecomastia occurs from an imbalance of the two sex hormones. Men have both androgens (the hormones that control hair growth, regulate muscle size, and aid in the development of a deep voice), and estrogens (the hormone responsible for female characteristics). So, in general, estrogen levels are higher than androgens, causing a moderate increase in breast size. It is important to understand that over half of the male population develop gynecomastia during puberty.
Sometimes, gynecomastia is a result of side effects of certain medications, a natural decrease of testosterone in older males, or associated with diseases or metabolic disorders that affect male hormones.
