According to MedlinePlus, liver spots can be caused by sun or ultraviolet light exposure and aging skin. Other causes are unknown, and these spots become common after age 40. Mayo Clinic explains that tanning lamps and tanning beds can also promote the growth of liver spots.
Mayo Clinic says that ultraviolet light ramps up the production of melanin, which gives skin its coloration. After prolonged, excessive melanin production, liver spots show up where melanin has been produced in high amounts or where it has stuck together. MedlinePlus says that such spots are common on the backs of the hands, shoulders, forehead, face and forearms.