Beyond the outcomes, a landmark randomized trial offers new insight into why some of Montessori's most distinctive practices may matter.
Recently, we published a data story on Dr. Angeline Lillard's 2025 national randomized controlled trial of public Montessori preschool. Much of the attention has focused on the headline findings: stronger outcomes in several important areas and lower costs than traditional programs.
Those findings are important. But for Montessori school leaders, there may be an even more valuable story inside the study.
Beyond answering whether or not Montessori works, the research offers clues about why some of Montessori's most distinctive practices may matter and why families may need time to see their value.
At Montessori Data Lens, our goal is not simply to report findings. It is to make research visible. Through data stories, we help schools, educators, and families engage with the growing body of Montessori evidence and the questions it raises for practice.
Why This Study Matters
For decades, one of the most persistent challenges in Montessori research has been selection bias.
Families who choose Montessori often differ from families who do not. When Montessori students demonstrate positive outcomes, critics have reasonably asked whether those outcomes reflect the educational model itself or characteristics of the families who selected it.
Dr. Lillard's study addresses that challenge through lottery-based admissions.
Published in the Proceedings of the National Academy of Sciences, the study followed 588 children across 24 public Montessori schools nationwide. Because admission was determined by lottery, researchers could compare children who were offered Montessori seats with children who applied but were not offered one.
By the end of kindergarten, children offered Montessori enrollment scored significantly higher in reading, executive function, short-term memory, and theory of mind. The study also found that three years of public Montessori cost districts less per child than traditional programs serving the same age range.
What makes these findings especially noteworthy is that several of the gains are not tied to a specific academic subject. Executive function, memory, and theory of mind influence how children focus, regulate behavior, reason through problems, and understand others. These are capacities that support learning broadly throughout life.
Because of its randomized design, the study provides some of the strongest causal evidence available on public Montessori education.
But the outcomes are only the beginning of the story.
What School Leaders Should Notice
Most readers stop at the results table. The discussion section offers insight into several Montessori practices that schools often struggle to explain to families.
The Three-Year Cycle May Matter More Than We Think
One of the study's most interesting findings is when the gains appeared.
Researchers found little difference between groups at the end of PK3 or PK4. The strongest advantages emerged by the end of kindergarten.
This pattern is particularly interesting because it aligns with one of Montessori's foundational structures: the three-year cycle.
The authors suggest that a child’s third year in the same Montessori environment may play an important role. Montessori educators have long observed that children often "blossom" during their final year in the classroom, consolidating earlier learning and assuming leadership roles within the community. The study points to previous research suggesting that completing the third year may be especially beneficial.
For schools, this finding carries an important implication.
Montessori may not function like a short-term intervention where results appear immediately. Some of its benefits may emerge only after children have had sufficient time to grow into the environment and experience the full cycle of learning.
For parents deciding whether to continue into a child's final year in a Montessori classroom, this is an important piece of evidence.
Why Larger Montessori Classrooms May Work Differently
Another insight comes from one of the most common questions Montessori schools receive from families:
"Why are Montessori classrooms so large?"
Many parents assume smaller class sizes automatically produce better outcomes. The study invites a more nuanced conversation.
The researchers note that Dr. Montessori herself advocated for larger classroom communities and suggested that learning may benefit from having more children together.
The reason lies in how Montessori classrooms operate.
Children learn not only from teachers but also from observing peers, receiving help from older students, and eventually becoming teachers themselves. Larger communities create more opportunities for mentoring, modeling, collaboration, and peer learning.
The researchers even suggest that tutoring younger classmates may help explain why Montessori advantages become more visible during the kindergarten year. Teaching others is itself a powerful form of learning.
What can appear to outsiders as an unusually large class is part of the educational design.
Children Drive Their Own Learning
The study also offers insight into another central Montessori principle: independence.
The authors propose that Montessori children may continue progressing because learning is not dependent solely on teacher-directed instruction. In Montessori classrooms, children engage directly with materials, pursue self-chosen work, and learn from peers as well as adults.
As children become more capable of directing their own learning, they may become less dependent on constant teacher attention.
This idea sits at the heart of Montessori education.
The goal is not simply to teach children information. It is to help them become capable learners who can pursue knowledge independently.
For many families, that distinction is difficult to see in the early years. Research like this helps explain why Montessori often prioritizes processes that may not produce immediate visible results but aim to build long-term learning habits.
Some Developmental Processes Take Time
Perhaps the most Montessori explanation in the entire study comes near the end of the discussion section.
The researchers suggest that some Montessori benefits may simply require time to emerge. Developmental processes can occur slowly and, as they describe it, may happen "underground" before becoming visible in measurable outcomes. Features such as self-directed learning, freedom within limits, and embodied experiences may take time before their effects become apparent.
Many Montessori educators will recognize this observation immediately.
Teachers often witness growth that is not yet visible in traditional academic measures. Confidence, concentration, self-regulation, independence, and social maturity frequently develop gradually before becoming visible to observers.
The study suggests that the same may be true at the population level.
The Fidelity Question
Another detail deserves attention.
The study did not focus on model schools. To be included, a school only needed to meet a set of minimum criteria, and the research team actually relaxed its original standards to broaden the pool of eligible schools. The final inclusion criteria were:
- At least 66% of lead Primary teachers trained by AMI or AMS
- No more than two adults (a trained teacher and a non-teaching assistant) in the classroom
- Mixed-age classrooms with at least 18 children ages 3–6
- At least a 2-hour uninterrupted work period each day
- At least 80% of the ~150 Montessori Primary materials, with fewer than 5% non-Montessori materials
This matters because Montessori implementation varies.
The Montessori term is not trade-marked, therefore the Montessori name alone does not guarantee that core elements of the model are present. The authors are explicit about what this implies. Their hypothesis was that Montessori would show significant benefits, but because they studied "a range of Montessori implementations (meeting minimum standards), those outcomes might be tempered relative to findings from studies using schools with strong implementation of the model." Prior lottery studies at AMI-certified schools — with higher-quality implementation — did indeed find strong effects.
This is what it means for your school. If you are an accredited, high-fidelity Montessori, you likely exceed every one of these minimums: fully trained guides, the complete three-hour work cycle, complete materials, true multi-age classrooms. The national study found significant, large gains at minimum fidelity, and the researchers themselves expect stronger implementation to produce stronger outcomes. So an authentic school has good reason to view these results as a floor rather than a ceiling.
For school leaders, that reinforces a practical lesson: fidelity remains one of the most important variables schools can control. The stronger a school's implementation, the more confidence it can have that children are experiencing the model as intended.
The question that stays with us
The study helps answer an important question: What outcomes can Montessori produce?
But as evidence about Montessori outcomes grows, a different question comes into focus.
Research can tell us whether a program works on average. It cannot tell us whether a parent understands why their child's classroom looks different, why mixed-age groupings matter, or why progress may not resemble traditional schooling. Yet those questions often shape a family's experience of Montessori.
The study helps us understand outcomes. But parents rarely experience Montessori through outcome data. They experience it through their child. That raises a different question:
How do we help families understand the daily learning experiences that sit behind the outcomes the research measures?
That question became the starting point for our thesis, Closing the Intention Gap.
The thesis began with a simple observation: schools often approach their parent education programs through philosophy, methodology, and educational principles, while parents are often trying to understand something much more immediate: what does Montessori mean for my child?
That difference creates what we call the Intention Gap: the distance between what schools intend for parents to learn and what parents most want to understand.
Rather than asking how schools could provide more parent education, the thesis explores a different question: could Montessori understanding be embedded into the context parents already care most about—their own child's learning experience?
That idea became the foundation for Montopia, our ongoing effort to explore how parent education can travel alongside a child's classroom journey, helping families connect everyday learning experiences to the larger Montessori principles and outcomes behind them.
Making the invisible visible
For us, Montessori Data Lens and Montopia are the same idea applied at different scales: making the invisible visible. Montessori Data Lens helps people understand a movement through research and data stories; Montopia helps families understand a child's learning through everyday classroom experiences.