Gynecomastia is triggered by hormonal imbalance, where there is a decrease in testosterone compared with estrogen.
Testosterone controls male traits such as muscle mass, body hair etc, whereas estrogen controls female traits such as the growth of the breasts. Men produce estrogen in small quantities, but when estrogen levels are out of balance with testosterone levels, this leads to gynecomastia.
Adolescent gynecomastia, which is caused by hormone changes during puberty) is common. Adult gynecomastia is prevalent amongst men ages 50 and above.