By Jon Hurdle

PHILADELPHIA (Reuters) - America's Amish population has nearly doubled and spread out in the past 16 years due to large families, more marriages within the community and longer lifespans, a study showed on Wednesday.

The population grew 86 percent to 231,000 in 2008 from 125,000 in 1992, or 4 percent a year, and is set to double from this year's level by 2026 if the current growth rate continues, according to the study by Donald Kraybill, a sociology professor at Elizabethtown University in Pennsylvania.

The Amish, a Christian sect that migrated to the United States from Europe in the 18th and 19th centuries, refuse to drive cars, use computers or connect to a public electricity supply. They speak a German dialect and travel through their predominantly rural communities in horse-drawn buggies.

At a time when some other ethnic and religious groups fear dilution through mixed marriages, the Amish have boosted their numbers by insisting on marriage within the group and providing education at Amish-only schools for 90 percent of their children, said Kraybill.

Their rate of population growth has accelerated in the past 20 years because they have an average of five or six children per family, and have done a better job of retaining their young people, he said.

The Amish -- whose dress is characterized by straw hats and suspenders for men and bonnets and long dresses for women -- largely avoid marrying outside their community because they know doing so would mean having to leave the church, Kraybill said.

"They will be excommunicated if they marry outside," he said.

In a sign of the spreading population, Amish numbers more than doubled in 10 states, and there was an 82 percent increase in the number of Amish communities throughout the United States. There are now Amish communities in 28 U.S. states.

In Pennsylvania, Ohio and Indiana, the three states that together have most of the Amish population, their share of the total Amish population fell to 62 percent this year from 69 percent in 1992.

They have been drawn to other states by fertile farmland at reasonable prices, work in specialized occupations such as cabinet-making or construction, and rural isolation that allows them to maintain their lifestyle, according to the study.

Kraybill added that the Amish were living longer and had a lower-than-average child mortality rate due to better health care.

(Editing by Michelle Nichols and Xavier Briand)