Nearly half of South Korea's roughly 51 million citizens share just four family names — Kim (about 21.5 percent of the population), Lee (14.7 percent), Park (8.4 percent) and Choi (4.7 percent) — a legacy of the country's history: commoners were only formally granted surnames within the last 150 years, and many took those already held by nobility

In South Korea’s 2015 census, one in five people was named Kim: 21.5 percent of the population, or 10.69 million people. Next came Lee at 14.7 percent, Park at 8.4 percent and Choi at 4.7 percent. Add those four together and you approach half a country of roughly 51 million. Stretch the list to the ten most common surnames and you cover 63.9 percent of everyone, barely down from 64.1 percent in 2000.

Picture that in a Seoul subway carriage. A dozen people named Kim might be sitting together, none of them related. So why does a country of tens of millions run on such a short list of names?

The answer sits mostly in the last two centuries of Korean history.

Why the pool of names is so small

For most of the Joseon dynasty, a surname was a sign of rank, not something everyone had. Family names belonged to the yangban, the aristocracy. Commoners and enslaved people generally had none. The story of Korea’s few surnames is really the story of how everyone below the aristocracy eventually got one, and which names they reached for.

The spread began long before any law required it. A study of one prefecture’s census records, reported by Moon Ki-hoon in The Korea Herald, found that the share of local households without a surname fell from 45 percent in 1681 to just 6 percent in 1816.

The 1894 turning point and the rush to noble names

The legal end of the class system came with the Gabo Reform of 1894. When surnames finally became universal, the people newly free to choose one tended to pick from a small set of high-status clans. Kim, Lee and Park were the obvious picks, and for a clear reason.

Andrew Eungi Kim, a professor at Korea University’s Graduate School of International Studies, told ESPN that “those names were the most prestigious for having produced large numbers of kings, royalties and other historically significant figures.” In a society where family line shaped your opportunities, choosing such a name carried real weight. The top three surnames now cover 44.6 percent of the population, a share that has barely moved since the Joseon era.

Once taken, those names held firm. Yumi Moon, a historian of East Asia at Stanford University, explained the taboo against dropping an inherited name: “Changing your surname [which is] inherited from your ancestors is disgraceful, according to the custom of the olden.” A name chosen for advantage in one generation became, in the next, an ancestral inheritance nobody would think to change.

Living with a short list of names

So many shared names creates practical problems, and Koreans built a workaround into the system: the bon-gwan, or ancestral seat. Two people can both be named Kim yet belong to completely separate clans, each traced to a different founding region. The largest, the Gimhae Kim clan, has around 4 million members, with the Gyeongju Kim adding roughly 1.5 million more.

That distinction once mattered in law. An old rule banned marriage between two people who shared both a surname and an ancestral seat. In practice, millions of unrelated Gimhae Kim could not marry one another. The Constitutional Court ruled the ban unconstitutional on 16 July 1997. A revised Civil Code, narrowing the ban to close blood relatives, took effect on 31 March 2005.

Custom has proved harder to shift than law. Kim Beom-jun, a professor at Sungkyunkwan University, notes that “In Korea, the Confucian tradition still runs deep with people, an example of which is the practice of not allowing their children to use different surnames from those of their fathers.” He does not expect the mix to change: “I don’t think there will be many changes in the portions of family names in the future, either,” he said.

There is one quiet source of new variety. The 2015 census recorded 5,582 distinct surnames, more than 4,800 of them newly registered since 2000. Most came from naturalised citizens writing foreign names into Hangul. The list is growing at the edges even as its centre holds. Whether a handful of clans can keep anchoring the identity of a slowly diversifying country is the open question the census leaves behind.