When George Washington started the first Presidency of the United States of America, he was governing less than 4 million people who occupied an area of 2,300,000 square kilometers. It was an agrarian society, with 95% of the population living on farms and only 5% in cities or towns of more than 2,500. There was plenty of land, and a vast array of natural resources, just waiting to be tapped. Today, we live in a country of over 294 million people. While we are one of the major food producers in the world, it is no longer due to us being an agrarian society. Today, most people live in towns and cities, with less than 25% of the population living in rural areas. And even though we have grown to a physical size of over 9.8 million square kilometers, our population density has increased from the 2 people per square kilometer of Washington’s day to almost 30 people per square kilometer today. From where did all of these people come? The increase in territory that we experienced over that time did increase the number of people in the U.S. from the simple fact that there were already people living there. We have also expanded by immigration, with wave after wave of people entering this country to find new life and new opportunity. Neither of these, though, accounts for the largest segment. Both put together still account for less than 80 million people. The largest sector of people is here due to birth. This same thing is true of any country. Population growth over time is a function of more people being born than dying each year, or to put it scientifically, it is a function of birth rate minus the death rate. Understanding what affects the death rate is pretty simple. Wars, famine pestilence, and disease all tend to increase the death rate, while abundant food, medicine, sanitation, and peace all tend to decrease it. But understanding what affects the birth rate is not quite as simple. To understand this, you have to understand why people have babies.