In nationally representative data from the Head Start Family and Child Experiences Survey 2014–2018 (FACES 2014), we found that, on average, first-year children showed larger gains in language and cognitive scores from fall to spring of the program year than second-year children (Kopack Klein et al. 2018). One explanation might be that first-year children are younger and have more skills to learn, but even when comparing first- and second-year children who were the same age when they entered Head Start, second-year children made smaller gains in some language and cognitive scores (Kopack Klein et al. 2018). In this brief, we use data across one year of Head Start (from fall 2014 to spring 2015) from FACES 2014 to expand this finding and explore possible explanations for why second-year children made smaller gains in language and cognitive scores than first-year children during the Head Start program year (see methods in the box at the end of the brief). (author abstract)