Indeed, the growth of population in the future United States in 18th century was astonishing. By 1730, the population of all 13 colonies was about 655,000 people; however, only 45 years later the amount of colonists reached 2.5 million (this is not to mention black slaves, who were also numerous). One of the reasons of such demographic boom was the conditions the colonists lived in. Experiencing the same difficulties as labor people in the United Kingdom (although rather typical for the 18th century) - poverty, epidemics, and so on - the colonists still enjoyed a higher quality of life: better food, cheaper land, bigger wages, more freedom. There were problems with the British taxations and representation in the Kingdom’s Parliament (actually, too many taxes with little to none representation), but for the most part, Britain did not interfere much in the colonies’ internal businesses, so common people could feel themselves comfortable and safe under the protection of the most powerful fleet, and having some trading benefits. Another reason for such a dramatic population growth was probably traditions and religion. The colonists had different origins, and many of them were convicts, smugglers, and so on, but still, the majority consisted of loyal and respected farmers, firmly believing in God and his laws, living by the traditions, observing the “be fruitful and increase in number” precept. And finally, do not forget that contraception was not really popular back then.