In 1964, at independence, Zambia's economic future looked brighter than that of most other developing countries. Its copper production accounted for 8% of total world production, and only neighboring Zaire outpaced it in the production of cobalt. Its Central Province around Kabwe held rich deposits of both zinc and lead; uranium deposits also had been found, but their projected yield remained undetermined. Since 1974, the decline in the price of copper and the increase in the price of oil have played havoc with Zambia's balance of payments. Copper, which accounted for 40% of the gross national product (GNP) and 98% of all foreign exchange in 1964, shrank to 12% of the GNP in 1978 while still generating most of the foreign exchange. As a result, imports were cut back markedly from $1.5 billion in 1973 to $690 million in 1983. Although this trend is beginning to make a U-turn, Zambia's economic situation is grave. In 1984 the GNP continued to register negative growth and inflation stood at 25%. With its urbanization rate doubling from 21% in 1964 to 43% in 1985, Zambia is now the most urbanized country south of the Sahara. Zambia's 1985 population is estimated to be 6.8 million. Between 1963 and 1969, the average annual population growth rate was 2.5: it was 3.1% between 1969-80. The current birthrate of about 48/1000 is expected to decline only marginally in the next 15 years, but the death rate is declining more rapidly -- from 19/1000 in the late 1960s to 15/1000 in 1985. Life expectancy is expected to rise from the current 51 years to about 58 years. As a result of the high growth rate, Zambia's population is young, with a median age of about 16.3 years. Traditional African values stress the importance of large families. Zambia's total fertility rate was 6.9 in 1985. According to the World Bank, only 1% of married women of childbearing age in 1982 used contraceptives. Although tribal links are weakening, Zambia still counts 73 officially recognized tribes. Together, they speak about 40 different dialects. Zambia now apportions over 15% of its national budget to education. Despite some noticeable progress, the public health structure remains deficient. Principal health problems include malaria, tuberculosis, and, in Northern Province and Luapula Province, sleeping sickness and river blindness. About 2/3 of the labor force, an estimated 2.2 million persons in 1982, still work in agriculture. Female labor force participation is lower in Zambia than in many African nations.