“Most probably it was Sigmund Freud’s influential essay on Leonardo’s homosexuality and Freud’s consequential analysis of the Mona Lisa which was the direct or proximate impetus for Duchamp’s image. But, whereas Duchamp seems to imply that the picture fuses artist and sitter, male and female, Freud suggests that the Mona Lisa (specifically her smile) is a manifestation of Leonardo’s submerged memory of the birth mother from whom he was estranged at age four and who Freud theorizes expressed an unnatural affection toward her young son. In fact, Freud refutes the notion that there is a physiognomic similarity between the artist and the sitter, but goes on to suggest that the device of the smile was obviously so meaningful to the artist, using it frequently in his works of the time, it must have repressed significance. The person behind the Mona Lisa, Freud suggests, may have had such a smile, a smile that evoked long ago suppressed memories of his mother. Indeed, as Freud is quick to point out, this seems to have been a persistent theme: Vasari even noted that at the earliest age Leonardo was known for having created images of smiling women:
Let us leave the physiognomic riddle of Mona Lisa unsolved, and let us note the unequivocal fact that her smile fascinated the artist no less than all spectators for these 400 years. This captivating smile had thereafter returned in all of his pictures and in those of his pupils. As Leonardo’s Mona Lisa was a portrait, we cannot assume that he has added to her face a trait of his own, so difficult to express, which she herself did not possess. It seems, we cannot help but believe, that he found this smile in his model and became so charmed by it that from now on he endowed it on all the free creations of his phantasy.
“(Sigmund Freud, Leonardo da Vinci: A study in psychosexuality. tr. A.A. Brill. New York, Vintage Books,  Originally published by Freud in 1910, p. 79.)”