Efforts to enroll more Maryland and Virginia 4-year-olds in state-funded preschool have seen positive but pricey results in the past eight years, according to a new study.

In Virginia, enrollees have more than doubled since 2001 for a total of about 13,100, says a report released Wednesday by the National Institute for Early Education Research. In Maryland, where state law requires preschool services be offered for all low-income 4-year-olds, the numbers have gone up by about half, to 26,800.