The way Lai Jie sees it, there are two kinds of relationships.
There is romance. And there is marriage.
They are not always the same, and in a city such as Shanghai, the deciding factor on who to wed can have little to do with personality, interest or chemistry.
It comes down, instead, to a little brown book, the hukou (pronounced hoo-koh) document, part of a household-registration system that creates distinctions between urban haves and have-nots that influence salaries, education and, it turns out, love lives for huge numbers of migrants.
In China, families are registered by hometown. Each person's hukou document anchors them to their family's place of origin and the services available there no matter where they move.
It means that those who move to big cities chasing jobs and opportunities typically do not enjoy full local rights of home ownership or benefits such as public schools. Without local hukou documents they are, effectively, second-class citizens – and despite Chinese efforts to reform the system, it remains largely in place today.
Many of the effects of hukou are well-known: It bars migrant workers in big cities from public services and education, and creates a difficult bureaucratic obstacle to personal advancement.
But it also plays a surprisingly important role in love, forming a major relationship barrier, creating a cleavage that calcifies social mobility and solidifies an under-class of citizens with curtailed rights.
Having hukou is "very important for the ones who don't have it," Ms. Lai said. "We must consider married life and then having children, unless it's a romance not aimed at marriage."
It's a problem reflected in new research from the University of British Columbia and Brown University. It found that although migrants make up just under half the population of Shanghai, only 20 per cent of marriages there cross hukou lines. Most unions are either between two people who possess Shanghai hukou, or two people who do not.
It is what researchers call " assortative mating," a dry term that describes an important social issue for China.
"Marriage and China's hukou system can work together to contribute to the growing socio-economic disparities between migrants and locals," said Yue Qian, a UBC professor who is the study's lead author. "Hukou just offers little chance for migrants, especially for less-educated migrants, to integrate or prosper in urban cities."
Conversely, it enhances the ability for those with Shanghai hukou to leap beyond their station.
"Local Shanghainese, with their hukou status, may still be able to marry relatively highly educated migrants. They can use their hukou to gain socio-economically through marriage," Prof. Qian said.
What it means is "migrants fare even worse in China compared with immigrants in Canada or the U.S.," she said.
"Children born to immigrants can get citizenship status automatically, as long as they are born in the U.S. or Canada," she said. "But in the Chinese context, even children born to migrant parents – those children are still migrants. They cannot gain Shanghai or Beijing hukou by birth."
China's hukou system is almost as old as Communist rule.
In 1958, Beijing required people to be classified as "agricultural" or "non-agricultural" (since amended to "urban" and "rural") along with their place of registration. The system is complex, but in general ties people to the homes of their ancestors, a practice that meant little in the agrarian China of a half-century ago.
But its persistence in modern days, with a quarter-billion Chinese who have left home to find better futures, has created numerous stresses.
It is on display at Shanghai's weekly marriage market, where parents seeking spouses for their children regularly advertise their hukou status. Shanghai hukou is considered the hardest in the country to obtain.
Chinese families are famously open about partnership requirements, bluntly asking potential mates about incomes, jobs and house holdings. Marriage offers one route to local registration, and the benefits it confers – spouses can, after waiting several years, apply to join each other's hukou.
But the persistence of hukou divisions underlie worries about class stratification and social mobility that have simmered in China. In 2011, Cai Zhiqiang, a professor at the powerful Central Party School, wrote an article lamenting that "the momentum for upward social mobility is being gradually lost," creating a situation where "hereditary poverty has become a reality."
Recent scholarship has borne that out. A Stanford study last year showed that a modern Chinese son's earnings were more likely to resemble his father's than those of someone in Brazil, the United States, Pakistan, South Korea or Canada, an effect researchers called a "very high level of intergenerational rigidity."
A 2014 study by Chinese and British researchers pointed out that on the scale of decades – dating back to before Communist rule in 1949 – China's upward mobility has exceeded that of Britain. But great class divisions remain, and "the prime driver for social inequality in China was the hukou system."
It can be surprising, then, to discover that hukou remains popular in China, even among those who recognize its role in maintaining social separation.
Take Ms. Lai, who cites former leader Deng Xiaoping's exhortation to "let some people get rich first." That has happened, and those living in places such as Shanghai "who got rich first also now enjoy the fruits of being rich first," Ms. Lai said. Hukou keeps those still poor from descending upon wealthy areas and taking away all that fruit.
That is not a bad thing, she said. "If Shanghai opened up its hukou policy, the city would be unable to bear such a large population," she said. "China has too many people. That is a fundamental fact."
Ms. Lai, a manager at a financial company, moved to Shanghai and married a man with local hukou – although his registration, she says, was not the primary attraction.
For many young Chinese today, though, hukou continues to weigh heavily as they venture into the marriage market.
Often, the primary consideration in choosing a spouse is home ownership. That, too, is tightly tied to hukou – which, scholars say, is the single-largest obstacle to buying a home for those not locally registered.
"Some with local hukou feel superior to others," said Anny Yuan, who recently graduated from a Master's program in economic management at Shanghai Jiao Tong University, one of the top education institutions in the country.
"For example, if one person has hukou, even if he doesn't have much in savings, he may expect the other half to be wealthier. The way I see it, it's a kind of marriage-market ploy."
Ms. Yuan is not from Shanghai, but her boyfriend is. She, however, has her own path to local hukou, which is granted more generously to top graduate students.
Even for those with Shanghai credentials, meanwhile, there is value in seeking out a spouse with a similar background. Life in Shanghai is not easy: Rents are high, jobs are fiercely sought after and square footage is small.
Most couples need both of them to work, meaning they rely heavily on grandparents to take care of children – which is much simpler when the older generation lives in the same city.
Cultural issues matter, too. Some of the city-born women who attend workshops run by Shanghai relationship consultant Wu Di are not interested in partners whose rural backgrounds may be geographically and culturally distant from their own.
In fact, Ms. Wu encourages this.
"It's not a question of hukou," she said. "Locals are better suited to marry other locals, in my opinion."
But, she said, marriage is complicated – and hukou is just one of a list of reasons that might make one person more attractive than another. Looks matter, as do wallet and house sizes.
"The more advantages one has, the easier it is for him or her to find their other half in marriage," she said.
With a report from Yu Mei
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