For the second time in three years, more people left Hong Kong last year than settled in the city, prompting warnings of a new brain drain. The mid-year provisional figures from the Census and Statistics Department showing net emigration of 12,400 people last year should be a 'wake-up call', one population expert said. Emigration exceeded immigration just seven years in the past 50. While previous exoduses could be attributed to events in the region, last year's figures have baffled experts. Chung Kim-wah, a social scientist at Polytechnic University, said there was no obvious reason for the high level of emigration last year, although he pointed to the introduction of the Hong Kong Diploma of Secondary Education as a possible cause. Many pupils had chosen to study abroad amid uncertainty about the new exam and concern about competition for university places with others in the double-size cohort resulting from the educational reform. A census department spokeswoman said population change was difficult to explain, as it included the movements of non-permanent residents, including domestic helpers, expatriate workers and overseas students. Security Bureau statistics show that 8,300 permanent residents left the city last year, up 15 per cent from the previous year. Professor Paul Yip Siu-fai, a population expert at the University of Hong Kong, warned that many of the people departing were professionals and members of the middle class. 'This is really a wake-up call,' Yip said. He called on the government to take steps to ensure there would be no 'structural brain drain', in which emigration exceeds immigration over a period of years. But Frederick Ho Wing-huen, former commissioner of the Census and Statistics Department, had another explanation for the phenomenon, which may cast new light on the debate over mainland women giving birth in Hong Kong. Those new mothers typically took their children back to the mainland after birth, Ho said, and about 60 per cent then sent their children back to the city for their education. Ho said the Hong Kong government should welcome the children and prepare for their return, saying: 'If you can't get rid of them, why don't you make them valuable?' There were several years of net emigration in the 1960s, in the early years of the Cultural Revolution. More people left than arrived in 1964 and 1969, and in 1966, more than 46,000 more people emigrated than immigrated. Net migration appears to have been prompted in 1984 by the signing of the Sino-British Joint Declaration, in 1990 by the Tiananmen crackdown the year before, in 2003 by the severe acute respiratory syndrome outbreak, and in 2009 by the economic downturn due to the financial crisis. The government is expected to release official population figures soon. In June last year, Hong Kong's population stood at 7.1 million, up 0.6 per cent from the year before. High levels of emigration could aggravate the city's long-standing problem of a low birth rate. Hong Kong's birth rate is just 1.04 per woman, prompting concern about a possible shortage of labour. The population is boosted by the arrival of up to 150 new residents a day from the mainland under a quota scheme intended to reunite families.