The British Nationality Act (1981) had in effect prevented Hong Kong citizens from acquiring British citizenship, and thousands of people, anxious about their future under China's rule, were prompted to apply for citizenship elsewhere, notably in Canada and Australia.
The British Nationality Act led to more people staying in Hong Kong. 
mnli hypothesis: The British Nationality Act led to more people staying in Hong Kong.  premise: The British Nationality Act (1981) had in effect prevented Hong Kong citizens from acquiring British citizenship, and thousands of people, anxious about their future under China's rule, were prompted to apply for citizenship elsewhere, notably in Canada and Australia.