Why One in Five South Koreans Is Called Kim

South Korea's 2015 census counted 10.69 million people called Kim. That is 21.5 percent of the country. Add Lee at 14.7 percent, Park at 8.4 and Choi at 4.7, and four surnames account for very nearly half the population.

The reason Kim sits at the top is the opposite of what you might guess. It got there by being exclusive.

A name that meant nobility

Kim was a clan name of the Korean aristocracy. The Gimhae Kim trace themselves to the royal line of Geumgwan Gaya, and the Gyeongju Kim descend from the ruling family of Silla, which puts the name at the centre of the peninsula's oldest political history.

Under Goryeo and then Joseon, that mattered enormously. Korea ran a hereditary class system with the yangban aristocracy at the top, controlling land, office and prestige. Most people below them either held no formal surname at all or carried one that marked them as outside the elite. Carrying Kim announced, however distantly, a connection to a throne.

What happened when the system collapsed

Joseon's class structure came apart during the reforms of the 1890s, which abolished the old registries and the slave system along with them. A hierarchy that had been effectively impossible to climb stopped existing.

People did what people generally do when status suddenly becomes available. They reached for it.

Genealogical records had been the documentary proof of aristocratic legitimacy, and once the class system fell away those records became something to acquire. Families bought them. Families forged them. Families with no aristocratic connection whatsoever attached themselves to prestigious clans on paper and adopted the surnames that went with them, which meant Kim, Lee and Park above all others.

Within a few generations a name that had marked a thin slice of the population covered a fifth of it.

Hundreds of Kims, not one

Korean surnames come paired with a bon-gwan, an ancestral seat that identifies which clan line a family belongs to. A Gimhae Kim and an Andong Kim share a surname and nothing else. Well over a hundred distinct Kim clans are recognised today, each with separate records and its own founding story.

The Gimhae Kim is much the largest, at around 4.46 million people in the 2015 count. The rest range from substantial to tiny, and some of them were assembled fairly recently by families who took the name and then picked a plausible seat to attach it to.

This custom used to carry real weight. Korea long discouraged marriage between people sharing both a surname and a bon-gwan, on the reasoning that they probably shared an ancestor somewhere. The rule loosened as the surname pool concentrated, for the practical reason that ruling out every fellow Kim removes a fifth of the country from consideration.

Kim in Central Asia

The surname travelled further than the peninsula. Ethnic Koreans settled in the Russian Far East through the late nineteenth and early twentieth centuries, and in 1937 Stalin's government deported them en masse to Central Asia, moving whole communities thousands of miles into what are now Kazakhstan and Uzbekistan.

Their descendants, the Koryo-saram, still live there, and many are still called Kim. The name outlasted the deportation, the change of country and in many families the loss of the Korean language.

What the name tells you

Very little about ancestry, which is the point. Kim spread because it was worth claiming, so the modern population of Kims is a mixture of old aristocratic lines and families who adopted the name during a window when the paperwork could be arranged.

Surnames get treated as biological facts. Korea's most common one is closer to a record of what people wanted to be during a period when that suddenly became negotiable.