Every child deserves individual attention. Research consistently shows that reducing class sizes leads to better outcomes — especially for the students who need it most.
Research consistently shows that smaller class sizes in early elementary grades (K–3) improve academic achievement, particularly for low-income and minority students.
Early grades show the strongest cognitive and social-emotional returns.
Small classes (≤18 students) in K–3 produce statistically significant improvements in reading and math. Effects are strongest for disadvantaged students. Effect size: +0.15 to +0.25 SD (moderate impact).
Students in small K–3 classes show higher long-term earnings, higher college attendance rates, and stronger non-cognitive skill development.
Class size reduction is effective but expensive. Most cost-effective when targeted to high-poverty schools.
Modest reductions (25 → 22 students) show little impact. Large reductions (25 → 15 students) show meaningful gains.
Average OECD elementary class size: 21 students. U.S. average: ~21–23 students. Countries with smaller early-grade classes often show higher early literacy outcomes.
Very large reductions (7–10 fewer students) can produce significant long-term effects, but policymakers must weigh CSR against alternatives. Teacher quality differences can be larger than class-size effects; implementation quality is decisive.
| Outcome | Impact of Small Class (K–3) |
|---|---|
| Reading Achievement | Moderate positive |
| Math Achievement | Moderate positive |
| Graduation Rates | Increased |
| College Attendance | Increased |
| Behavioral Outcomes | Improved |
| Teacher Retention | Improved |
| Earnings (Adult) | Increased |
“Students assigned to smaller classes are more likely to attend college and earn higher wages.”
— Dr. Raj Chetty, Harvard University
“Reducing class size in the early grades has a positive effect on reading and mathematics achievement.”
— Institute of Education Sciences
“Smaller classes are often perceived as allowing teachers to focus more on the needs of individual students and to reduce time spent on disruptions.”
— OECD Education GPS
From randomized experiments to international quasi-experiments, the academic evidence on class size spans decades and continents. Here are the key studies.
The gold-standard RCT (1985–89) randomly assigned 11,571 K–3 students across 79 Tennessee schools to small (13–17) or regular (22–25) classes. Krueger’s (1999) econometric reanalysis confirmed overall effect sizes of 0.19–0.28 SD across grades. Long-term follow-up (Krueger & Whitmore, 2001) showed small-class students were significantly more likely to take ACT/SAT (43.7% vs 40.0%), with adjusted score gains of ~0.13 SD overall and 0.20–0.26 SD for Black students. Chetty et al. (2011) linked STAR participants to adult records, finding significantly higher college attendance; earnings effects at age 27 were positive but imprecise. This “test-score fadeout with meaningful adult outcomes” pattern underscores that short-term test gains alone understate the full value of early class-size reductions.
Wisconsin’s Student Achievement Guarantee in Education program reduced class sizes to 15 students in K–3 at high-poverty schools. Results showed significant achievement gains across all three core subjects, with particular benefits for African-American students. Teacher reports confirmed improved classroom dynamics beyond test scores alone.
Gilraine, Macartney & McMillan (2020) evaluated California’s massive K–3 class-size reduction using 914,514 enrollment observations. Classes dropped from ~28.5 to ~19.5 students. Beyond direct math gains of ~0.11 SD, the study found that school composition changes amplified the benefits, making the policy more cost-effective than the direct effects alone would suggest.
Tanaka & Wang (2025) used student-teacher pair fixed effects and 2SLS with predicted class size as an instrument across 145,264 student-year observations in Japanese elementary schools (grades 2–6). The study found a statistically significant negative effect of larger classes on math scores, with a 10-student reduction yielding ~0.087 SD gains. Importantly, the study demonstrates that peer composition interacts with class-size effects — how classmates are grouped matters alongside how many there are.
Argaw & Puhani (2017) used administrative panel data from Hesse, Germany, covering 258,098 students. Using the cap-at-25 maximum class-size rule as an instrumental variable with school fixed effects, they found that larger first-grade classes significantly increase grade repetition through elementary school. Notably, this study shows that class-size effects on schooling progression emerge even in a system with already relatively small classes.
Fredriksson, Öckert & Oosterbeek (2013) used Sweden’s maximum class-size rule as an instrumental variable and followed students from primary school into their 40s. The study found that smaller classes in grades 4–6 significantly boost cognitive ability at age 13, with effects persisting into adult wages. Their cost-benefit analysis yielded an internal rate of return of ~18%, making this one of the most compelling long-run cases for class-size investment in the international literature.
Smaller classes can improve socioemotional and behavioral outcomes in early grades, but effects vary by context, baseline class size, and outcome measured. The biggest payoffs appear in early grades and for disadvantaged students.
| Outcome | Impact of Smaller Classes |
|---|---|
| Social Competence | +0.07 SD per 10-student reduction (K) |
| Emotional Maturity | +0.12 SD per 10-student reduction (K) |
| Communication Skills | +0.08 SD per 10-student reduction (K) |
| Student Initiative | +0.18 SD (Grade 4, after K–3 treatment) |
| Student Distress | Reduced (especially low-income) |
| Bullying Victimization | Reduced in high-class-size settings |
Connolly & Haeck (2021) used Quebec’s province-wide cap-at-20 rule as a natural experiment, applying fuzzy regression discontinuity / IV methods across 108,151 kindergarteners. Outcomes were standardized (mean 0, SD 1). The study found that reducing class size meaningfully improves teacher-rated social competence, emotional maturity, and communication skills — with effects concentrated in disadvantaged areas and strongest at smaller class sizes.
Beuchert & Nandrup (2025) used Danish administrative and school data to establish a causal link between class size and student well-being. The study found that larger classes in grades K–2 increase self-reported physical and emotional distress, with effects appearing strongest among children from low-income families — highlighting that class size policy has consequences for student welfare beyond test scores.
Hallaq (2020) used maximum class-size rules in West Bank schools (governmental max 40; UNRWA max 45/50) as a regression discontinuity instrument across ~6,000 students in grades 5–9. The study found that class size increases meaningfully worsen peer safety and behavior outcomes, with mechanism evidence linking larger classes to mental-health difficulties. The results imply that reducing very large classes can yield meaningful socioemotional benefits.
Dee & West (2011) reanalyzed Project STAR follow-up data, focusing on non-cognitive outcomes. Using observational controls and fixed effects across 2,212 students at Grade 4, they found that K–3 small-class assignment significantly improves teacher-rated initiative. While some socioemotional benefits exist, persistence and breadth across domains is not guaranteed — supporting a cautious but positive synthesis for early-grade interventions.
Blatchford, Edmonds & Martin (2003) conducted a large longitudinal study of English reception and Key Stage 1 classes, combining systematic classroom observations of 235 children across 39 classes with teacher-rated behavior scales for 5,000+ children. While observations clearly showed more off-task behavior in larger classes, teacher ratings of peer relations yielded mixed patterns — an important reminder that “social outcomes” can move differently depending on how they are measured.
Garandeau et al. (2019) conducted an observational multilevel study across 77 Austrian and 59 Dutch classrooms (2,911 students, mean ages ~11–12). The study found that bullying can appear higher in smaller classes, partly due to how peer-nomination measures work and social-status dynamics. This important caveat does not negate causal evidence from other settings, but highlights that bullying prevention requires more than class size reduction alone — social norms and bystander interventions matter too.
| Study | Year | Country | Method | Grades | Key Effect |
|---|---|---|---|---|---|
| Project STAR Word et al. | 1990 | US (TN) | RCT | K–3 | Reading 0.21–0.34; Math 0.17–0.33 SD |
| STAR Follow-Up Krueger & Whitmore | 2001 | US (TN) | Experimental | K–3 → HS | ~0.1 SD overall; ~0.2 SD Black; 56% gap reduction |
| SAGE Wisconsin | 1990s | US (WI) | State program | K–3 | Significant gains in math, reading, language arts |
| California CSR Gilraine et al. | 2020 | US (CA) | Policy eval. | K–3 | Direct math: ~0.11 SD |
| Japan Tanaka & Wang | 2025 | Japan | FEIV / 2SLS | 2–6 | Math: ~0.087 SD per 10-student reduction |
| Germany (Hesse) Argaw & Puhani | 2017 | Germany | Quasi-exp. (IV) | 1–4 | Grade repetition: +0.4 pp per student |
| Sweden Long-Run Fredriksson et al. | 2013 | Sweden | Quasi-exp. (IV) | 4–6 | Cognition: +0.23 SD (7-student cut); IRR ~18% |
| Socioemotional & Behavioral | |||||
| Quebec K Connolly & Haeck | 2021 | Canada | Quasi-exp. (RD/IV) | K | Emotional maturity: +0.12 SD / 10 students |
| Denmark K–2 Beuchert & Nandrup | 2025 | Denmark | Causal | K–2 | Increased distress in larger classes (low-income) |
| West Bank Hallaq | 2020 | West Bank | Quasi-exp. (RD) | 5–9 | Bullying: +12.9 pp / 10 students |
| STAR Non-Cognitive Dee & West | 2011 | US (TN) | RCT reanalysis | K–3 → 4 | Initiative: +0.18 SD / 10 students |
| UK Reception/KS1 Blatchford et al. | 2003 | UK | Longitudinal | Reception | More off-task behavior in larger classes |
| Austria/Netherlands Garandeau et al. | 2019 | AT / NL | Observational | Pre-adolescent | Bullying higher in smaller classes (measurement effects) |
Class combining and size increases are usually driven by resource constraints and enrollment dynamics — not pedagogy. Understanding these drivers is essential for effective advocacy.
When revenues fall or costs rise, districts reduce staffing or avoid adding sections. Loss of funding or budget cuts cited by 52% of schools that lost teaching positions.
Unfilled roles force schools to cover classrooms with fewer adults. 29% of schools reported class sizes increased as a result of vacancies.
When enrollment drops, per-pupil funding formulas lead to fewer sections. Decreased enrollment cited by 55% of schools that lost positions.
When students arrive above projections, schools may temporarily exceed targets, combine sections, or delay adding a new classroom/teacher.
Even with funding, schools may lack physical classrooms. 31% of public schools have portable buildings in use; 21% reported major repair work.
When teachers are absent and substitutes are unavailable, schools combine classes or distribute students. More than 75% of schools reported greater difficulty getting substitutes.
Special education, bilingual/ESL, and other specialized vacancies trigger reallocations that inflate general classroom sizes. 74% of schools reported difficulty filling special education positions.
Ironically, class-size laws can increase combination classes: schools may combine in some grades to keep capped grades compliant, or apply for exemptions permitting larger classes.
Maintaining electives, specialty classes, intervention blocks, or required services can reduce available general-ed sections, raising average class sizes.
When systems face simultaneous underutilization and overcrowding, merging schools or reconfiguring grade bands changes class sizes across the board.
Average primary class sizes across OECD countries and trends over time. Essential for international context.
A clearinghouse of studies, reports, and external links on class-size effects.
Quick-reference fact sheets for advocates, parents, and policymakers.
Critical analysis and meta-review of the class-size reduction literature.
Comprehensive summary of STAR, SAGE, and international evidence.
Whitehurst & Chingos on evidence, cost-effectiveness, and when large reductions matter most.
Federal Class-Size Reduction program design, implementation lessons, and K–3 target of 18 students.