A. Gynecomastia is an increase in male breast tissue. This is something which is different from "chunkiness" or obesity as there is an increase in the glandular tissue of the breast itself.
Gynecomastia is a normal finding in up to 70% of boys going through puberty and is likely to go away on its own after 1 to 2 years (it's usually due to an increased production of estrogen by the testes at that time). We see it in about 1% of adult men.
As for causes, it's 1 of several reasons. First of all, breast tissue will enlarge or develop in response to the hormone estradiol (a "female" hormone) . So, any condition that will increase estrogen production or decrease androgen (the "male" hormone) will result in gynecomastia. On a more technical note, anything that will block androgen receptors (the place on the cell where androgens first have an effect on the cells they effect) or bind them up in the blood so less is available to affect cells, will do the same.
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