Andrew Batson, China research director for Gavekal Dragonomics, notes that the 2020 census implies a much sharper decline in the populations of three northeastern provinces than previously believed. Since 2010, the recorded populations of Liaoning, Jilin and Heilongjiang have declined by 2.6%, 12.3% and 16.9%, respectively.

The National Bureau of Statistics monitors the selling prices of homes in five of those provinces’ largest cities: Shenyang, Dalian, Harbin, Changchun and Jilin City. And in the past five years, newly built residential property prices have risen by at least 40% in each of them. Properties are far cheaper than in the wealthiest cities, but the climb in prices hardly reflects the reality of a sliding provincial population.

There is a major qualifier to the data, of course: The story in many smaller urban areas is almost certainly much bleaker. Comprehensive, up-to-date granular data on population sizes isn’t available. But that reinforces the point that speculative activity can continue in the bigger cities—where most of it takes place already—for quite some time after a province’s demographics turn sharply negative.

A study published last year looked at vacancy rates in Chinese cities, based on nighttime light data. According to the estimates, vacancy rates run to around 22% in Dalian, Harbin, Changchun and Shenyang—above the already-high 15% and 16% seen in Shenzhen and Shanghai, respectively.

Chinese real-estate markets’ ability to sustain both very high vacancy rates and surging prices is excellent evidence of speculation. And since speculative behavior isn’t always based on actual economic conditions, there is no immediate reason to believe that demographic shifts will cool the housing market—at least in big cities.

Rents in the northeast, which are more directly determined by actual supply and demand for lodging, tell a different story. In Harbin and Dalian, monthly rents per square meter have barely budged in the past decade—in contrast to the approximate doubling in tier one markets like Shanghai, Beijing and Shenzhen.

Demographics could eventually become a key factor in the big Chinese housing markets. But right now, even a falling national population wouldn’t necessarily be an impetus for rapid change.

Write to Mike Bird at Mike.Bird@wsj.com