This is the research companion to The Problem With Up and to the Right. Read the main post first.
The main post asks what a falling score can tell us about development. A measurement captures what a child can do under particular conditions; understanding the change takes more work. These four research papers and one book examine why performance can fluctuate, why earlier learning can interfere, and why a skill may need to be learned again in a new context.
The Sampling Study
Adolph, K. E., Robinson, S. R., Young, J. W., & Gill-Alvarez, F. (2008). What is the shape of developmental change? Psychological Review, 115(3), 527-543.
DOI: 10.1037/0033-295X.115.3.527
What they found: Thirty-two infant motor skills were recorded every day across the first 18 months. Then the researchers simulated what a study would have seen by checking only every 2 to 31 days, deleting the days in between. The daily record was full of fluctuation, skills appearing, vanishing, and returning for weeks before they settled. Sampled less often, the same data turned into clean step functions, and estimated onset ages drifted off target. The loss of sensitivity followed an inverse power function, with severe degradation once the gap between checks passed seven days.
Why this matters for you: How often researchers looked changed the developmental story they saw. For parents, it is a reason to describe the pattern, including when a skill appears and disappears, rather than relying on two isolated observations.
What it doesn’t answer: Infant motor milestones from parent checklists. It says nothing directly about reading fluency or homework tolerance, and it is a paper about how researchers sample, not about how parents watch.
The Past-Tense Errors
Marcus, G. F., Pinker, S., Ullman, M., Hollander, M., Rosen, T. J., & Xu, F. (1992). Overregularization in language acquisition. Monographs of the Society for Research in Child Development, 57(4), 1-182.
DOI: 10.2307/1166115
What they found: 11,521 irregular past-tense utterances from 83 children. Errors like goed and comed were rare, a median of 2.5 percent of irregular past-tense forms, and an extended stretch of correct use came before the first error. The errors first appeared when children began reliably marking regular verbs for tense, when “Yesterday I walk” became “Yesterday I walked.”
Why this matters for you: The goed that sounds like backsliding arrives with the rule. The child acquired more grammar, not less. And the error rarely takes over; it sits inside speech that is mostly right.
What it doesn’t answer: These were English-speaking children, and the errors were uncommon. A past-tense error is a specific example of change during learning, not a reason to dismiss other losses of language skills.
The Nine-Year-Old Dip
McNeil, N. M. (2007). U-shaped development in math: 7-year-olds outperform 9-year-olds on equivalence problems. Developmental Psychology, 43(3), 687-695.
DOI: 10.1037/0012-1649.43.3.687
What they found: On problems like 7 + 4 + 5 = 7 + __, performance in a first study of 87 children fell between ages 7 and 9 and rose again between 9 and 11. A second study of 35 children replicated the 7-to-9 decline. The proposed mechanism is change resistance. Years of arithmetic in one format, operations on the left and the answer on the right, build patterns strong enough to interfere when the format changes, until later math loosens them.
Why this matters for you: The findings suggest that familiar arithmetic patterns can interfere with solving a differently shaped problem. More experience does not guarantee better performance on every task.
What it doesn’t answer: One problem type, in American classrooms teaching conventional arithmetic. Different children at each age rather than the same children followed, so the U is a pattern across groups, and the mechanism is inferred from the pattern rather than measured directly.
The Crawl-to-Walk Reset
Adolph, K. E. (1997). Learning in the development of infant locomotion. Monographs of the Society for Research in Child Development, 62(3, Serial No. 251), 1-158.
DOI: 10.2307/1166199
What they found: Infants were tested on slopes from their first week of crawling until several weeks after they began walking. Their judgments grew more accurate with crawling experience, but that learning did not transfer to walking. They had to learn how to navigate slopes from an upright position. What they had learned depended on their way of moving and their vantage point.
Why this matters for you: The same slope became a new problem when a child began walking. The main post uses that finding as an analogy for school transitions, when new conditions can make an earlier competence harder to use.
What it doesn’t answer: Infants and slopes. The middle-school extension is ours, an analogy the study supports in shape but does not test.
The Overlapping Waves
Siegler, R. S. (1996). Emerging Minds: The Process of Change in Children’s Thinking. New York: Oxford University Press.
ISBN: 978-0-19-507787-2
What they found: Across arithmetic, memory, and problem-solving tasks, children at any given age use several strategies for the same problem, not one. Development is a gradual shift in the mix, older strategies fading while newer ones gain, rather than a clean replacement. Variability itself tends to come before advance; the children using more strategies at once were often the ones about to learn.
Why this matters for you: Using several strategies can be part of learning. A child may be trying different ways to solve a problem before one becomes dependable; variability alone does not tell us that learning has stopped.
What it doesn’t answer: A model of thinking strategies built mostly in lab tasks. Stretching it to emotional regulation or getting ready for school is our extension, and the book synthesizes many studies rather than reporting one.
Coming Up
This post opens a new slate, and part of it is not written yet. If this post found you in the middle of something, tell one of us about it. Many of the posts here started exactly that way. We answer.
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