The financialization of art (by Axios)

Art is a much tougher asset class to invest in than watches. Investment-grade art starts at about $500,000, and liquidity is much lower since artworks are unique. The round-trip costs involved in buying and then selling a work of art can easily exceed 30% of the value of the work, while the bid-offer spreads on a Rolex accompanied by all of its authenticity papers are much lower.

Between the lines: Art has a much longer history as an asset class than watches do, and so it's easier to see financial trends emerging.

How it works: The Mei Moses art index, which was bought by Sotheby's in 2016, gauges price appreciation by looking at repeat pairs of sales — situations where the same object is sold at auction more than once.

• The time between such sales is a good indication of how long collectors hold onto art. And that number has now reached an all-time low.

The big picture: Even old-school collectors regularly buy and sell as they hone their collections. Condé Nast patriarch Si Newhouse, for instance, owned the "Shot Sage Blue Marilyn" for about 10 years, from the early 1970s to the early 80s.

• Most collectors, however, hold onto most of their high-end art until they die, at which point it's often donated to a museum. Financial speculators, by contrast, buy in order to sell at a profit in the future.

The bottom line: The pictures that show up in the Mei Moses database are by their nature unusual: Most paintings are never sold at auction even once, let alone twice. But if the holding period for works at auction is declining, it seems reasonable to conclude that's also true for art in general.