Introduction
 
In the United States, many young children attend an array of preschool programs that vary widely in their cost to families, funding sources and administration (public or private), standards, and goals.  Roughly 75 percent of all young children attend preschool at age four and half of these children attend preschool at age three (Barnett, 2011).  Unfortunately, most programs are not of high quality (Karoly, Ghosh-Dastidar, Zellman, Perlman, & Fernyhough, 2008).  Among children in low-income families, participation in effective preschool programs is about 10 percent at age three and 20 percent at age four (Barnett & Lamy, in press). Children in higher-income families have somewhat better access to quality preschool programs.  However, most children even from families that fall in the top half of the income distribution do not attend good preschool programs as judged by standardized observation systems (e.g., an Early Childhood Environment Rating Scale of 5 or higher) (Karoly, Ghosh-Dastidar, Zellman, Perlman, & Fernyhough, 2008).  State standards for the education of young children have risen over the past decade, but vary considerably by state. Most young children do not have access to public Pre-K (Barnett, Epstein, Carolan, Fitzgerald, Ackerman, & Friedman, 2010). 
 
Preschool or school entrance does not signal the beginning of learning.  Children learn from the start of life, and the home environment plays a role in this learning (Duncan et al., 2007; Melhuish et al., 2008; Zimmerman et al., 2009).  More importantly, there are differences in literacy experiences in the home, such as how many books a family owns, and how often parents or caregivers read to their children.  These differences are related to social class differences (Burchinal et al., 2011; Duncan & Magnusson, 2005).  Experiences with language have also been shown to differ for young children before the start of school.  For example, children from professional families hear more than three times the amount of words, with a higher level of vocabulary, than children from low income families (Hart & Risley, 1995).  These early experiences are directly related to cognitive differences at Kindergarten, and the resulting differences persist well beyond school entry (Burchinal et al., 2011; Dickinson, 2011; Hart & Risley, 1995).  Preschool has a positive effect on early cognitive development and has been shown to minimize these early differences. 
 
Key Research Questions

1. Why should language and literacy learning be a key focus during the preschool years?
2. What is the impact of preschool on language and literacy learning?
3. Do the effects of preschool education on language and literacy learning last?
4. What are the economic benefits of preschool education? 

Recent Research Results
 
Why should language and literacy learning be a key focus during the preschool years?
 
Early literacy skills found at preschool-age have been shown to have a moderate to strong relationship with later literacy skills such as decoding, comprehension, and spelling.  Both a meta-analysis of close to 300 correlational studies (NELP, 2008) and an analysis of existing data from previous large-scale studies (Duncan et al., 2007) provide strong evidence that early precursors to literacy such as alphabetic knowledge, phonological awareness, letter sounds, concepts about print, and rapid naming are highly related to later literacy proficiency in Kindergarten and beyond.  Although important, the relationship of these “code skills” with later literacy learning should not overshadow the importance of language development in the early years.  There is strong evidencethat language ability at ages three and four has strong predictive power for literacy skills in the early grades through high school (Camilli, Vargas, Ryan, & Barnett, 2010; Dickinson & Porche, 2011; NICHD et al., 2005; Verhoeven, van Leeuwe, & Vermeer, 2011).
 
The brain develops rapidly in early childhood, creating a specific “window of opportunity” that is sensitive to experiences that establish certain capabilities.  Genetic studies indicate that only approximately one third of the variability in language and later reading is determined by genetic factors (Dickinson, 2011).  This leaves ample room for the effects of intervention, creating a moderate to large impact on early literacy skills and later achievement (NELP, 2008).  The flip side to this sensitive period where interventions have the potential to provide the most substantial impact is that it is also the time where the brain is most highly susceptible to the absence of these critical experiences.  This can have lasting detrimental effects.  Children who enter school substantially behind their peers are unlikely to catch up.  It has been shown in elementary school that gaps in test scores across socioeconomic groups are stable by age, suggesting that later schooling has little impact in reducing the gaps that appear before students enter school (Duncan et al., 2007; Heckman, 2006).  Furthermore, this early skill development provides the foundation for later conceptual knowledge.  It also creates an interdependency of early learning, which magnifies the importance of providing a strong foundation before Kindergarten entry (Dickinson, McCabe, & Essex, 2006; NICHD, 2005; Shonkoff & Phillips, 2000) and establishes the need for high-quality preschool.  
 
What is the impact of preschool on language and literacy learning?
 
Individual state Pre-K program evaluations have demonstrated strong effects of preschool on language and literacy skills at Kindergarten entry (Gormley Jr., Gayer, Phillips, & Dawson, 2005; Hustedt, Barnett, Jung, & Friedman, 2010).  When examined together, although the programs differed on several aspects, data from eight state preschool programs added to the evidence demonstrating that high-quality preschool can have uniformly large effects on emergent literacy and vocabulary (Barnett, Jung, Frede, Hustedt, & Howes, 2011).  Most recently, the evaluation of Tennessee’s Voluntary Pre-K Program (Lipsey, Farran, Bilbrey, Hofer, & Dong, 2011) utilized two different research methodologies to examine the effects of the program.  Both methods found that the strongest gains in language and literacy were for children who attended the program versus those who did not.  The fact that these two study designs came to the same conclusion lends credibility to both studies, strengthening the results. 
 
Do the effects of preschool education on language and literacy learning last?
 
Several well-known longitudinal studies of the effects of early intervention demonstrate a relationship between preschool experiences and later achievement.  The HighScope Perry Preschool Study (Schweinhart et al., 2005) looked at the effects of a two-year half-day preschool program (with home visits by the teachers) and found moderate effects on reading and language achievement through adolescence.  The Chicago Child-Parent Center Program Study (Reynolds, 2000) found positive effects of comprehensive educational and family support services for economically disadvantaged children’s reading achievement through high school, demonstrating over a five-month gain in reading achievement at age 15 over those who did not participate in the program.  The Abecedarian study (Campbell et al., 2002) examined the effect of a time-intensive education program and found large positive effects on reading achievement through age 21.  A recent meta-analysis (Camilli, Ramey, Pungello, Sparling, & Miller-Johnson, 2010) of the most rigorous studies of the impact of Pre-K found that initial effects of interventions equaled closing approximately 70 percent of the achievement gap.  Although the size of the impact decreased over time for longitudinal studies, the effects of Pre-K intervention were still quite notable beyond Kindergarten. 
 
However, not all interventions have been found to be as equally effective as those described above.  The National Impact Study of Head Start (Puma et al., 2010) and the evaluation of Early Head Start (Vogel, Xue, Moiduddin, Carlson, & Kisker, 2010) found little to no initial effect on language and literacy and the small effects found did not persist to first grade for literacy-related measures. 
 
Why the conflicting findings?  Program features such as teacher qualifications and compensation may have some impact, although this is unlikely to alone have an effect on outcomes (Barnett, 2011).  The decrease of the initial impact may be the effect of compensatory efforts of high-quality public school experiences focused on children entering school with lower skills.  Thus, what is observed is “catch-up,” rather than “fade out” (Barnett, 2011).  Also, it has been demonstrated that the attributes and quality of the intervention are a factor in determining the effect of early education and care on cognitive outcomes (Sylva et al., 2011; Vandell et al., 2010).   For example, the amount of small group instruction (Camilli et al., 2010) and the quality and quantity of language used in the classroom (Dickinson & Porche, 2011) have been shown to be directly related to the effect of the intervention on cognitive outcomes. 
 
What are the economic benefits of preschool education? 
 
Preschool programs that produce lasting positive effects on young children’s language and literacy development, as well as on other domains, including socio-emotional development, are valuable investments in our future.  The use of the term investment is not just a rhetorical device.  Three rigorous studies have found that the benefits of high-quality preschool programs greatly exceed their costs and produce high rates of return compared to even private sector financial opportunities (Barnett & Masse, 2007).   All three studies followed children through school and into adulthood. Although two studies focused on small-scale model programs, the third focused on a large-scale preschool program operated by the Chicago public schools (Reynolds, Temple, White, Ou, & Robertson, 2011). 
 
The estimated economic benefits of preschool education derive from an array of short- and long-term impacts (Barnett, 2011).  These include effects on grade repetition and special education rates, educational attainment and earnings, and delinquency and crime.  There is also evidence of improved mental health and heath-related behaviors (e.g., smoking) (Barnett, 2008).  In addition, programs providing sufficient hours of child care support created more opportunities for parents to obtain employment, therefore, increasing earnings by parents (Barnett & Masse, 2007).  Increased parental income also has positive benefits for young children’s development, though these are small relative to the direct benefits of high-quality preschool education.  The evidence that preschool programs produce these short- and long-term effects comes from a large international body of research.  Thus, the case for a high rate of return ultimately rests on a much broader set of studies than the three with formal benefit-cost analyses.   
 
How important are the economic benefits that might be expected from public programs generally?  The three formal cost-benefit analyses produce benefit estimates of around $100,000 to $300,000 per child (Barnett & Masse, 2007).  Costs also vary substantially.  The Chicago program is closest to typical practice for “high-end” public programs and its study suggests benefits around 10 times cost (Barnett & Masse, 2007). Even if programs had only 10 to 20 percent of the Chicago program’s effectiveness (because the program was weaker, the population is less in need, or the Chicago results are for other reasons atypical), benefits would still be in the $10,000 to $20,000 per child range.  Such benefits are large enough to justify one or two years of high-quality full-day Pre-K on purely economic grounds.
 
Conclusions and Future Directions
 
Several conclusions can be drawn from early language and literacy development research and the impact preschool education can have on these outcomes.  One, language and literacy development in the early years is important and should remain a focus for researchers, policymakers, and practitioners.  Two, there is substantial supporting evidence, although varied based on program characteristics and quality, to demonstrate that preschool education has a strong effect on cognitive development and that this can lower the differences currently found at school entry.  Third, there are meaningful longitudinal effects of preschool and these effects are mediated by the quality of the intervention.  Lastly, preschool education can provide long-lasting improvements in school achievement that demonstrate economic benefits, such as higher educational attainment and earnings and lower arrest rates for those who attend programs.   
 
Further research is needed as we investigate ways to improve preschool education and identify specific factors that increase the effect of preschool on children’s development and performance.  First, a closer look at specific program features and teacher pedagogy that play a role in producing substantial effects on children’s cognitive development is needed.  In this research, there should remain a focus on specific sub-groups (i.e., children with special needs, English Language Learners, varying SES and other differing family characteristics) as we move forward with large-scale, longitudinal evaluations, using randomized control trials to examine the lasting impact of preschool education.