Writing hand preference and cognitive function across the childhood years: Findings from the Millennium Cohort Study

[call.me.meg]

Are left-handers or right-handers smarter? A study of >10,000 children in the UK has the provocative answer.

In this sample, southpaws and right-handers differed in a few important ways ways. Lefties were more likely to be:
Male
White
Children of mothers with higher levels of maternal stress.
Right- and left-handers were similar in all other background characteristics.

On the cognitive scores, there were no statistically significant differences at age 3, but differences (favoring right-handers) started to emerge at age 5 and generally got larger at older ages. The largest differences were in spatial abilities, where right-handers outscored other children by about d = .11 to .15. (Note that in the table below, lower scores on the spatial working memory task indicate better performance.)

The differences are too small to notice in daily life, though. Most of the distributions for the cognitive variables look like this the graph below. This study provides information that would be useful to theorists in neuroscience and experts in handedness. But has few (if any) practical implications.

Link to original post: https://x.com/RiotIQ/status/1976649670463287339?s=20

Link to full article: https://doi.org/10.1016/j.intell.2025.101952

[Marcelo]

This is solid work - large sample, longitudinal design, and transparent about effect sizes. The key finding is that the differences are real but practically meaningless (d = 0.04-0.15). I appreciate that they’re upfront about this: useful for theory, irrelevant for practice. The spatial skills pattern is the most interesting part, especially since it emerges and strengthens across the school years rather than being present from the start.

[CloverL]

The developmental timing is what stands out to me - no differences at age 3, then effects appearing and growing through age 11. If this were purely about brain lateralization, you’d expect consistent differences from early on. The fact that it emerges during schooling suggests environmental factors might be at play. But with effect sizes this small (d ≈ 0.11-0.15 at most), it’s really just noise in any practical sense.