In 1315, when the Venetian city-state was at the height of its economic powers, the upper class acted to lock in its privileges, putting a formal stop to social mobility with the publication of the Libro d'Oro, or Book of Gold, an official register of the nobility. If you weren't on it, you couldn't join the ruling oligarchy.
The elites imposed austerity on the ecnomic system of one of the most innovative and productive societies in Europe. By doing that they killed the goose that laid the golden egg.
The political shift, which had begun nearly two decades earlier, was so striking a change that the Venetians gave it a name: La Serrata, or the closure.
It wasn't long before the political Serrata became an economic one, too. Under the control of the oligarchs, Venice gradually cut off commercial opportunities for new entrants. Eventually, the colleganza was banned.
The reigning elites were acting in their immediate self-interest, but in the longer term, La Serrata was the beginning of the end for them, and for Venetian prosperity more generally.
By 1500, Venice's population was smaller than it had been in 1330.
In the 17th and 18th centuries, as the rest of Europe grew, the city continued to shrink.