[1] Univ N Carolina, Dept Publ Policy, Chapel Hill, NC 27599 USA
[2] Univ N Carolina, Frank Porter Graham Child Dev Inst, Chapel Hill, NC 27599 USA
[3] Univ Maryland, Dept Human Dev & Quantitat Methodol, College Pk, MD 20742 USA
[4] Boston Coll, Lynch Sch Educ, Chestnut Hill, MA 02167 USA