Researchers at the Rand Corp. find not only are children of poorer, less-educated mothers less likely to be enrolled in preschools, but few preschools promote the thinking and language skills that prepare children for kindergarten.

The study finds 45 percent of children with mothers lacking a high-school diploma attend center-based preschool programs, compared to 80 percent of children of mothers with graduate degrees.

The study, based on 2,000 households, said one in four children participates in a preschool classroom providing quality instruction. The children from more-affluent families are no more likely to experience high quality environments than children from low-income families.

"It is now the norm for California's 3- and 4-year-olds to spend at least part of their day in a center-based early care and education program," lead author Lynn Karoly, an economist at Rand Corp., said in a statement. "Unfortunately, relatively few of the centers we studied provide the types of high-quality early learning experiences that can help prepare children to succeed when they enter school."