What else has nearly doubled? The percentage of girls who are home-schooled. They now outnumber home-schooled girls by a wide margin.

As of spring 2007, an estimated 1.5 million, or 2.9% of all school-age children in the USA, were home-schooled, up from 1.7% in 1999.

The new figures come from the U.S. Department of Education, which found that 36% of parents said their most important reason for home schooling was to provide "religious or moral instruction"; 21% cited concerns about school environment. Only 17% cited "dissatisfaction with academic instruction."

Perhaps most significant: The ratio of home-schooled boys to girls has shifted significantly. In 1999, it was 49% boys, 51% girls. Now boys account for only 42%; 58% are girls.

That may well be a result of parents who are fed up with mean-girl behavior in schools, says Henry Cate, who along with his wife home-schools their three daughters in Santa Clara , Calif . "It's just pushing some parents over the edge," says Cate, who writes the blog Why Homeschool.

Home schooling has grown most sharply for higher-income families. In 1999, 63.6% of home-schooling families earned less than $50,000. Now 60.0% earn more than $50,000.

Cate says many highly educated, high-income parents are "probably people who are a little bit more comfortable in taking risks" in choosing a college or line of work. "The attributes that facilitate that might also facilitate them being more comfortable with home-schooling."

Among the other findings:

•3.9% of white families home-school, up from 2% in 1999.

•6.8% of college-educated parents home-school, up from 4.9% in 1999.

Michelle Blimes home-schools her three daughters in Orem, Utah . Initially it was for academics, and now she sees social benefits. "They should be able to enjoy playing and being kids before being thrown into the teen culture," Blimes says.