According to a survey carried out by Census Bureau, the nation's largest and fastest-growing minority group are the Hispanics. About 15% of the U.S. population is Hispanic and this is mainly the result of births among the people already in the country, not of the immigration as many would have thought.

The data provided by the Census Bureau shows that there are 45.5 million Hispanics in the U.S., up from 35.7 million in 2000. The Hispanics are responsible for half of the U.S. population growth between 2000 and 2007.

The tendency has changed. In the 90’s, the Hispanic immigrants were the main cause of the ethnic group’s population gains, but during 2006-2007 approximately 62% of the increase in Hispanics came from births.

"The Hispanic population has taken on a momentum of its own. If you close the borders tomorrow, there is still going to be a large Hispanic increase," said Kenneth Johnson, senior demographer at the University of New Hampshire's Carsey Institute.

More than 50% of the children aged under 5 in New Mexico and California are Hispanic. In other states with a large Hispanic population such as Texas, Arizona, Nevada and Colorado, the situation is quite similar. About a third of the children aged less than 5 are Hispanic.

According to the study, the Hispanic immigration to the United States has actually decreased from July 1, 2006, to July 1, 2007, compared to the previous year. Experts believe the main cause for the immigration drop is the United States' economic slowdown.

White women in U.S. give birth to 1.8 children, slightly under the replacement rate of 2.1 necessary to maintain a stable population, while the Hispanic women have 2.8 children on average, the census shows.