During my thirty years as a professional art historian, this or a very similar story has been published about once a decade, which is about long enough a new generation of scientists to think it has hit on something novel and for a new generation of journalists to decide it's a headline story for the silly season. They ignore the far greater significance of social traditions and craft skills.
The overwhelmingly majority of portraits were painted in Europe in oils between 1420, when the necessary skills had been developed, and 1900, when they had been largely superseded by photography.
Most were commissioned by powerful and arrogant men. When a woman was portrayed it was usually because she was somebody's wife.
A man portrayed on his own might face left or right. But is was established as early as 1434, when Jan van Eyck painted the so-called Arnolfini double portrait, now in the National Gallery, that the wife is shown on her husband's left hand. So, even when on separate canvases, the portrait of the wife is painted to hang to the right (as seen by the viewer) of that of the man. Each turns to face the other; it would look very odd if they did not.
A further detail: a right-handed painter would choose to have the light coming over his left shoulder, so that his working hand did not cast a shadow over the area he was painting. Therefore, his sitter, facing him, is illuminated from the sitter's right. In consequence, in those (mostly male) portraits with the sitter turning left, the bridge of the nose is emphasised by a strong shadow while the nose of a (woman) sitter turning to her right is more softly defined.
Wivenhoe, Essex.Reuse content