history question What caused the population of the United States to more than double between 1790 and 1820? A. an explosion of immigration. B. natural increase. C. a population explosion and increased immigration. D. a massive increase in the slave trade. Solution Between 1790 and 1820, the population of the United States had more than doubled. This growth was almost entirely the result of reproduction, a natural increase. The immigration rate, during this period was very low as less than 250,000 immigrants entered the United States during this period. There were doubts about the viability of the new republic. Also, during the French Revolution and Napoleonic Wars, there were travel restrictions in Europe. The immigration to the United States began to increase after Napoleon’s final defeat in 1815. The option B is the correct answer..