MIDDLE-CLASS families in the most prosperous regions in China are finding the least happiness in life, because of high stress levels in their daily lives, a major survey has found.

The happiness-index level in smaller cities, it turns out, is much higher than that of the country's biggest cities.

The much-publicised pinch of high housing prices, fierce office competition, traffic congestion and the high cost of kids' education were the primary causes of the low happiness index in big population centres such as Shenzhen and Beijing, the Guangzhou Daily newspaper reported on Wednesday.

More generally, despite improving health levels, better education and higher incomes, the middle class is discovering that economic pressures and minimal time to spend with family are major sources of annoyance, according to a survey by insurer Manulife-Sinochem.

"The middle class is a 'sandwiched class', being worse off than some and better off than many," said academic Yan Ye, a professor with the North China Institute of Science and Technology.

"The government should expand coverage of social security, promote low-income housing projects and adjust income distribution to help them," Prof Yan added.

The Manulife-Sinochem survey was conducted over two months last year and polled more than 70,000 people aged 20 to 40 years in 35 cities across China.

All the respondents reported yearly incomes above 50,000 yuan (S$10,205). Last year, China's per-capita gross national product was estimated at 25,000 yuan.

Some 200 million to 300 million people belong to the middle class in China, accounting for 23 to 25 per cent of the total population. Remarkably, that proportion has been increasing by 1 per cent each year, according to Prof Yan.

According to Demographic Transition of Contemporary China, a research report released by the Social Sciences Academic Press recently, the rapid growth in the middle class results from the swelling army of professionals entering the labour market, thanks to the growing number of graduate workers.

This has intensified - but not necessarily in a balanced manner - wealth accumulation amid the country's extended economic boom, the report said.

The Manulife-Sinochem survey results also revealed that respondents with annual household incomes from 110,000 yuan to 200,000 yuan were the happiest.

Also, people aged 30 to 35 years are generally happier than those in other age groups. No reason was cited for this finding.

Meanwhile, those living in second-tier cities experience more happiness than those in large cities, such as Beijing, Shanghai and Shenzhen.

Residents of Jiangsu, Sichuan, Fujian and Chongqing rank tops in terms of happiness, with 50 per cent of people satisfied with their present situation.

"Fifty thousand yuan a year might be good money for someone in a second-tier city. But that is not so in Beijing," said 23-year-old Beijinger Jian Biao.

According to Prof Yan, people usually have higher income expectations in big cities. So, this psychological factor means that they are more likely to lack a sense of security and to feel a sense of injustice.

Another academic suggested a different reason.

"As people become wealthier, they start to pursue a better quality of life," observed Professor Liu Jun at Shenzhen University.

"The transition from focusing on careers to the quality of life will bring about problems that disappoint people," he was quoted as saying by Guangzhou Daily.