You'd think it would be tough to cloak a nearly 1,900-foot-tall building in secrecy, yet that's exactly what's happening with the proposed China 117 Tower in Tianjin, China, and many other skyscrapers still in the proposal or early building phases. Which is why Philip Oldfield, a research coordinator at the Chicago-based Council on Tall Buildings and Urban Habitat, compiled a “Tallest 20 in 2020” list last November as well as a list of the 50 tallest proposed buildings in January, unveiling some of the mysteries for a greater collective understanding.

“We used strict criteria on real proposals that are moving forward,” says Oldfield. The Chicago Spire and 1 World Trade Center, both under way, are the only North American buildings on the 2020 list. “If you made this list 30 years ago, you'd have predicted the majority of the buildings would be steel office buildings in North America,” Oldfield says, adding that today's tallest buildings are primarily residential or mixed-use, made of concrete, and located in Asia and the Middle East.

Carol Willis, founder and director of the Skyscraper Museum in New York City, cites cultural differences as a deciding factor in where tall buildings go up. Zoning laws and building codes are cultural, not economic, she says.

Driven by a desire to create icons in a given city, the tall trend shows no signs of slowing down—at least overseas. But North America's lock on tall design is coming to an end. “It's not that we have a lack of ambition or money [in the United States]— that's not it,” Willis says. “There's a limited amount of space you can exploit in the sky. Whether by democratic process or not, it's been decided that the public owns a piece of the sky here.”