Pakistan has just more than half as many people as the United States but if the U.S. stopped accepting new international immigrants, at Pakistan's current rate, it will surpass the United States in population within 20 years. Pakistan is one of the world's most rapidly growing countries with an annual population growth rate of 2.4 percent. This rate of growth results in a doubling of a population every 30 years. Pakistan's population was a mere 32.5 million people when it won independence in 1947. In the sixty years since independence its population increased by more than 120 million to reach 166 million people. The current rate of growth for the United States, which is influenced by movement of a large number of people into the country, yields a doubling time of 120 years. For the U.S. and Pakistan, persistence of these same rates of population growth for 115 years would result in a staggering population of more than 2 billion for Pakistan At that time Pakistan would have a population that is 3.3 times that of the United States.
Toney, Michael & Berry, E Helen (2007). Population Change, in: Dolch, NA, et al. (eds.), Social Problems: A Case Study Approach, 2nd ed. pp: 251-274.