Tens of thousands of Hong Kong-born Canadian residents have returned to the SAR since 1996, according to an analysis by the South China Morning Post that helps establish the extent of an unprecedented phenomenon shaping both societies.
Canadian census and immigration statistics indicate an exodus of more than 65,000 Hong Kong-born residents in the 15 years to 2011. And more than 153,000 may have left from 1981 up to 2011, the analysis suggests.
However, this reverse migration is not even acknowledged by Hong Kong because it does not officially recognise dual citizenship. According to a November 2012 Hong Kong government assessment, there were only 16,554 Canadians in Hong Kong.
Prominent Canadian urban planner Andy Yan said that understanding the true scale of the flow was crucial to planning policies in both Hong Kong and Canada, which risked being stymied by the lack of hard data.
"How do we plan our cities? How many housing units, how many services do we need?" said Yan, an adjunct professor of planning at the University of British Columbia (UBC).
"This [reverse migration] is a massive X factor and is the frontier of urban planning."
The Hong Kong government regards Canadians as only the eighth most prevalent foreign residents in the city. But the Post's analysis, incorporating Canadian census data released this month, adds weight to the belief that Canadians are by far the most numerous foreign passport holders in Hong Kong.
This assessment is shared by Ottawa, which believes there may be 295,000 Canadians in Hong Kong, including Canada-born children of immigrants not included in the Post analysis. However, this estimate is based on a telephone poll, and Canada has no way of accurately tracking the whereabouts of its citizens.
Daniel Hiebert, a geographer at UBC who has studied reverse migration and advised the Canadian government on the issue, rejected the official Hong Kong assessment and said the scale of the return flow from Canada to Hong Kong was unmatched.
"The impact [of a lack of data] is that we don't exactly know how many of these folk who emigrate from Hong Kong to Canada stay, and it's hard to figure out where they live now. Do they mainly live here or there?" he said.
"For a start, this has a big impact on the housing markets. One family might have two or more dwellings, and that has a big impact."
A range of social policies, from schooling to health, hinged on understanding the scale of the flow, he said.
The Post's analysis is based on the most recent 15 years of Canadian data. From 1996 to the end of the first quarter of 2011, some 64,788 new immigrants from Hong Kong arrived in Canada. But in the same period, Statistics Canada data shows the population of Hong Kong-born residents fell instead by 31,320.
Adjusting for mortality leaves a 65,521-person gap between the number of Hong Kong-born people in Canada and the expected number of living émigrés.
Hong Kong migration to Canada exploded after the 1984 Sino-British joint declaration on Hong Kong. More than 300,000 Hongkongers migrated to Canada in the 1980s and 1990s, hitting an annual peak of 44,271 in 1994. But in 1998, after the handover, immigration plummeted.
Since 2001 there has been an average of fewer than 500 new Hong Kong immigrants arriving in Canada each year.
A source in Hong Kong's Information Services Department confirmed that the vast Canadian presence is not officially acknowledged because China's nationality law does not allow for dual citizenship.
Returnees from Canada feature prominently among Hong Kong's elite. They include tycoon Li Ka-shing, WHO director general Margaret Chan Fung Fu-Chun, actor Nicholas Tse Ting-fung, and a host of Miss Hong Kong winners including current title holder Carat Cheung.
How to pin a number on Hong Kong's floating Canadian population
Establishing the vast number of immigrants to Canada who have returned to Hong Kong has been a long-standing demographic mystery.
But instead of attempting to count the number of Canadians in Hong Kong, the analysis by the South China Morning Post calculates the number of Hong Kong immigrants absent from Canada, by comparing 15 years of immigration and census data.
There were 241,095 Hong Kong immigrants in Canada for the 1996 census, with 64,788 more arriving by the time of the 2011 census. But by then there were only 209,775 in Canada, a shortfall of 96,108 people.
Applying Canada's 0.7 per cent annual mortality rate to the existing 1996 immigrants and later arrivals suggests 30,587 deaths, and 65,521 Hong Kong immigrants who had left Canada. However, a lower death rate may apply since immigrants undergo rigorous health checks.
There have been no identifiable spikes in population or immigration of Hong Kong-born people to the US, Britain, Australia or New Zealand from 1996 to 2011, making it highly likely that the vast bulk of the 65,521 simply returned to Hong Kong.
The Post's calculation is imperfect. It cannot account for some immigrants from Hong Kong who might have been born in other parts of the world, or who might have migrated from Canada to mainland China.
Extending the analysis back to 1981, the number of returnees balloons to 153,247. But because the pre-1996 period coincided with large-scale Hong Kong emigration around the world, it is not possible to state conclusively that all these went back to Hong Kong.
The most authoritative estimate of the number of Canadians in Hong Kong to date was made in a 2011 study for the Asia Pacific Foundation of Canada, upon which the Canadian government's own assessment is based. It used a telephone poll of 35,825 Hong Kong households to find that 1,800 included at least one Canadian. From this, a "conservative estimate" of 295,930 Canadians in the city was extrapolated. That is more than twice the number of Americans and Australians combined, according to consular estimates.
The number of Britons in Hong Kong has never been truly established. But the population of Hongkongers in Britain varied little before or after the handover, standing at 72,884 in 1991, peaking at 94,611 in 2001 and estimated at 82,000 by the Office of National Statistics in 2011 .
Such slight variation rules out a large pool of British returnees to build on the 22,000 Britons reportedly present in the city during the handover . This excludes people with a British National (Overseas) passport, which does not convey full citizenship.