IN THE mid-1990s posters plastered on the subway in Seoul, South Korea’s capital, exhorted local girls to marry farmers. Young women had left their villages in droves since the 1960s for a better life in the booming city. Sons, however, stayed behind to tend family farms and fisheries.

The campaign was futile. Last year over a fifth of South Korean farmers and fishermen who tied the knot did so with a foreigner. The province of South Jeolla has the highest concentration of international marriages in the country—half of those getting married at the peak a decade ago. In those days, the business of broking unions with Chinese or South-East Asian women boomed, with matches made in the space of a few days. Not long ago placards in the provinces sang the praises of Vietnamese wives “who never run away”. Now, on the Seoul subway, banners encourage acceptance of multicultural families.

They are expected to exceed 1.5m by 2020, in a population of 50m. That is remarkable for a country that has long prided itself on its ethnic uniformity. But a preference for sons has led to a serious imbalance of the sexes. In 2010 half of all middle-aged men in South Korea were single, a fivefold increase since 1995. The birth rate has fallen to 1.3 children per woman of childbearing age, down from six in 1960. It is one of the lowest fertility rates in the world. Without immigration, the country’s labour force will shrink drastically.

The government is the biggest enthusiast for a multi-ethnic country. Its budget for multicultural families has shot up 24-fold since 2007, to 107 billion won ($105m). Some 200 support centres offer interpreting services, language classes, child care and counselling. School textbooks now include a section on mixed-race families. And in 2012 mixed-race Koreans could join the army for the first time. When four Mongolians working illegally in South Korea pulled a dozen Korean colleagues from a fire in 2007, locals urged the government to grant them residency (it did).

Still, assimilation remains elusive. Four in ten mixed-race marriages break down in the first five years, according to a survey by the Korean Women’s Development Institute, a think-tank. In 2009 almost a fifth of children from mixed-race households who should have been in school were not. Many mothers have limited Korean. And discrimination lurks.

The government is now tightening up the marriage rules. Last month two new requirements came into force: a foreign bride must speak Korean, and a Korean groom must support her financially. Koreans are now limited to a single marriage-visa request every five years.

Critics say making marriage more difficult will only serve to speed up the greying of the workforce. The pool of eligible women will shrink, says Lee In-su, a marriage broker in Daegu in the south-east. Most foreign brides come from rural areas lacking language schools. Meanwhile, competition for brides from China, where men also outnumber women, is fierce.

In fact, the number of Korean men taking foreign brides is dropping, from 31,000 a year in 2005 to 18,000 last year. And nine-tenths of matches are now urban, says Mr Lee. Vietnamese girls no longer want to languish in the Korean countryside, says Kim Young-shin of the Korea-Vietnam Cultural Centre in Hanoi, Vietnam’s capital. They like watching Korean dramas and listening to K-pop—urban pursuits.

As for Korean clients, says Lee Chang-min, a broker in Seoul, they are increasingly better educated and better-off; some are among the country’s top earners. Many are simply on lower rungs of the eligibility ladder in a culture captivated by credentials, whether in looks, age or family connections. Others, Mr Lee says, are wary of the stereotype of the doenjangnyeo (a disparaging term for a class of Korean women seen as latte-loving gold-diggers). They prefer a wife who can assume a more traditional role than one many Korean women are nowadays willing to play. These men, the brokers lament, are now more likely to be introduced to their foreign wives through friends than through brokers. Perhaps a modest win for melting-pot Korea after all.