Willibald Sauerlander, in the NYR article, notes of Messerschmidt that "There was always something unsettled about his biography, the life of an outsider." Apparently a talented sculptor, at some point his career veered off the path of acceptability and commercial success and he began to sculpt "head pieces" or, as th. schmid, a researcher with a quirky website devoted to the artist, calls them, "mad heads." Made of stone, pewter or lead, the heads are extraordinarily expressive busts, at once both comical and disturbing.
The NYR blog has a slide show of 10 of the heads.