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(CNN) -- The United States became a nation of 300 million people Tuesday morning, the U.S. Census Bureau estimates.
Fueled by an estimated net gain of one person every 11 seconds, America -- the world's third most populous country -- joined China and India with populations greater than 300 million.
The main reason is simple: Births outnumber deaths. According to the Census Bureau, a child is born every seven seconds, but a death occurs every 13 seconds.
But immigration also plays a role. The Census Bureau estimates that a migrant enters the country every 31 seconds.
America claimed 100 million people in 1915 but didn't reach 200 million until 1967. The 400 millionth person is likely to arrive in 2043, according to the Census Bureau.
Flight to South, West
Since 1967, the American population has undergone several demographic changes. Americans have moved out of the Northeast and Midwest and into the West and the South, according to the Washington-based Population Reference Bureau.
For instance, in 1967, Phoenix, Arizona, had a population just shy of 440,000, according to the Census Bureau. By 2006, the city had 1.5 million residents and was the sixth most populous in the nation. Florida in 1967 had a population of 6.2 million; the Sunshine State now boasts 17.8 million people.
"The West surpassed the Northeast in total population back in 2000 and is projected to overtake the Midwest region before 2030," said Linda Jacobsen, director of domestic programs for the Population Reference Bureau, told reporters Wednesday in an online discussion. "The South will continue to have the largest population of any region through 2030."
In addition, American suburbs have seen steady growth -- between 1970 and 2000, the percentage of the total population living in suburbs grew from 38 percent to 50 percent.
Despite the larger population, however, there are fewer large households in the United States. In 1970, less than 18 percent of households consisted of just one person. During the next 30 years, one-person households increased to nearly 26 percent of the total population.
The change is fueled by both young and old; young people may delay getting married and choose to live on their own, while older people who are divorced or widowed also live alone rather than remarry.
"As men and especially women live longer and are healthier and are able to live independently, the share of households that consist of a person living alone will likely continue to increase," Jacobsen said in a report tied to the attainment of the 300 million mark.
And classifying households is not as easy as it may have been in previous years, Jacobsen noted in the report. A household with a mother, a child and the mother's boyfriend, for instance, might not be classified as a family, the Population Reference Bureau said.
Meanwhile, married-couple households have dropped from nearly 75 percent in 1967 to 50 percent today. And nonfamily households have increased from 17 percent to 33 percent.
Other points noted by the Census Bureau include more women in the workforce and better-educated Americans.
Immigrants bypassing 'gateway states'
Migration is playing an increasing factor in population statistics and is projected to continue to do so. According to the Census Bureau, there were 9.7 million foreign-born people in the country in 1967. By 2004, that number had mushroomed to 34.3 million -- 12 percent of the total population.
"Immigration is certainly contributing to the increasing racial and ethnic diversity of the U.S. population," Jacobsen said Wednesday. "The share of the population who will be Asian and the share who will be Hispanic is projected to double between 2000 and 2050."
However, she said, "it is important to note that the share of the population that was foreign-born was higher [13 percent to 15 percent] during the whole period from 1860 to 1920 than it is today. One pattern that is different today is that immigrants are increasingly dispersed in communities across the U.S."
The Population Reference Bureau noted in its report that traditional "gateway" states such as New York, New Jersey, California, Texas and Florida have long been a "first stop" for immigrants.
"But increasingly, immigrants don't go to traditional gateway states at all," Jacobsen said. Some go directly to relatives in destinations such as North Carolina, Nevada and Georgia.
Although the majority of current immigrants are Mexican, Jacobsen said Wednesday that a proposal to build a fence along the Mexican border would not slow population growth -- "not in the short term."
"Hispanics in the U.S., especially Mexicans, have higher fertility rates than non-Hispanic whites," she said. "Even if no additional immigrants came across the border in the next few years, population would continue to grow in the U.S. because of this built-in momentum."
Impact on the environment
Although the nation's growth rate is larger than that of any other industrialized country in the world, it remains slower than that of developing countries, including India and China, she said.
The country's burgeoning population is having an adverse effect on the environment, Jacobsen said.
Land is being developed at twice the rate of population growth, and some of the nation's fastest-growing regions are in the Western dry areas, which affects water resources.
Air pollution is a problem in larger cities, she said, and poor air quality may contribute to increased health problems among children and the elderly.
Energy is also a concern; the United States consumes a quarter of the world's energy and is the single largest emitter of carbon dioxide in the world.