in the 1970s, 1980s and 1990s. The capital city of Seoul is also the country's largest city and chief industrial center. According to the 2005 census, Seoul had a population of inhabitants. The Seoul National Capital Area has inhabitants (about half of South Korea's entire population) making it the world's second largest metropolitan area. Other major cities include Busan (), Incheon (), Daegu (), Daejeon (), Gwangju () and Ulsan (). The population has also been shaped by international migration. After World War II and the division of the Korean Peninsula, about four million people from North Korea crossed the border to South Korea. This trend of net entry reversed over the next 40 years because of emigration, especially to North America through the United States and Canada. South Korea's total population in 1955 was , and has more than doubled, to 50 million, by 2010. South Korea is considered one of the most ethnically homogeneous societies in the world with ethnic Koreans representing approximately 96% of total population. Precise numbers are difficult to estimate since statistics do not record ethnicity and given many immigrants are ethnically Korean themselves, and some South Korean citizens are not ethnically Korean. The percentage of foreign nationals has been growing rapidly since late 1990s. , South Korea had 1,413,758 foreign residents, 2.75 percent of the population; however, many of them are ethnic Koreans with a foreign citizenship. For example, migrants from China (PRC) make up 56.5 percent of foreign nationals, but approximately 70 percent of the Chinese citizens in Korea are (), PRC citizens of Korean ethnicity. Regardless of the ethnicity, there are 28,500 US military personnel serving in South Korea, most serving a one-year unaccompanied tour (though approximately 10 percent serve longer tours accompanied by family), according to the Korea National Statistical Office. In addition, about 43,000 English teachers from English-speaking countries reside temporarily in Korea. Currently, South Korea has one of the highest rates of growth of foreign born population, with about 30,000 foreign born residents obtaining South Korean citizenship every year since 2010. Large numbers of ethnic Koreans live overseas, sometimes in Korean ethnic neighborhoods also known as Koreatowns. The four largest diaspora populations can be found in China (2.3 million), the United States (1.8 million), Japan (0.85 million), and Canada (0.25 million). South Korea's birth rate was the world's lowest in 2009, at an annual rate of approximately 9 births