It is only AFTER the Renaissance that portraiture really caught fire. The Italians pioneered many of their best portrait effects--chiaroscuro, the triangular composition, the use of the ova--and of course the use of oils which was one of the biggest factors in the rising popularity of the painted likeness. Oils let artists fret over every stroke, and rise to new heights in capturing new depths of the setter's appearance and personality. The Spanish added size to the portrait painter's repertoire. The scale of their full-length and group portraits still astounds us. And the Germans along with their English cousins across the channel added an elegance of detail not even imaginable a generation before Hans Holbien or Rubens. And the French contributed color, and texture, and PAINT to the genre. The American contribution to all this was simply to assimilate it all, as we are prone to do in other things, thus creating a sort of hybrid, super-portrait as seen in the work of John Singer Sargent.
However in this century, nothing much new happened to the portrait. Perhaps the advent of photography as the primary portrait medium caused the painted portrait to stagnate. That is until Pittsburgh-born artist Andy Warhol got hold of it. Single-handedly he had a greater effect upon the painted portrait than anyone since Leonardo made Mona Lisa smile (sort of). His pop icons exploded in the face of traditional portrait painting. The camera and the silk-screen were as much tools for him as the brush. Color became totally arbitrary. Elements of abstract design, rhythms and repetition began to dominate. A whole new set of portrait criteria had to be developed to even TALK about his bold departures from "accepted" portrait practices. Flat and shallow became positive attributes. The Post-modern portrait had arrived, and even though few have sought to follow in his painted footsteps, every portrait done either before or since seems to have a vaguely old-fashioned air about it.