Population growth in the Shenzhen Special Economic Zone (SEZ) is running out of control and threatens to hold back the city's development, a population expert has said. Yang Lixun of the Shenzhen Academy of Social Sciences was quoted by local media as saying the problem was caused by a large influx of migrants from other parts of China, most with little education. That could delay the SEZ achieving its goal of "modernization" defined as attaining economic standards comparable with those of developed countries – by another five years, Yang said. Shenzhen has a registered population of 4m, although including unregistered migrants it is thought to be closer 10m. The SEZ also has the highest population density of China's biggest cities standing at 3,597 people per sq km, compared to Shanghai's 2,902.