<P> Although there was relatively little immigration from Europe , the rapid expansion of settlements to the West , and the Louisiana Purchase of 1803 , opened up vast frontier lands . The high birth rate , and the availability of cheap land caused the rapid expansion of population . The average age was under 20 , with children everywhere . The population grew from 5.3 million people in 1800 , living on 865,000 square miles of land to 9.6 million in 1820 on 1,749,000 square miles . By 1840 , the population had reached 17,069,000 on the same land . </P>
what caused the areas of the united states to nearly double between 1800 and 1810
high birth rate availability of cheap land