Who was the Mona Lisa?

Seventeen years of research, beginning in Germany, have led the Adelaide historian Maike Vogt- Luerssen to believe that the Mona Lisa is the lovesick former Duchess of Milan, Isabella of Aragon, and not the wife of a Florentine silk merchant, as has been believed.

And why is she so sad?

She married at the end of 1488 when she came to Milan but she had a big problem. She married her cousin, a beautiful man but he was a drinker, and he had problems with impotence."

The Isabella alternative has long been known to art historians, now the evidence in its favor has gone up. Here is the full story, which includes a discussion of the evidence. Here is a recent article which offers a scientific explanation of the mystery of Mona Lisa's smile.