Field noteExplainerMetascienceVolume 3, Issue 1, Fall 2026

Two-Thirds of Cats Sleep on Their Left Side, Until Somebody Rechecked

A 2025 Current Biology report found 63.8% of cats sleeping left-side-down, framed as feline handedness. An erratum followed within months, then an independent three-platform recheck in 2026 found nothing. We line up both papers and ask what a 'preference' has to mean before a single photo can prove one.

Written jointly by the Science Journaling Club

Study: Field note
Review: Peer-edited by the club review board
Length: 13 sectionsabout 18 min
Files: LaTeX source22 kB
Interactive companion

Abstract

In June 2025 a thirteen-author team led by Sevim Isparta coded 301 YouTube videos of sleeping cats and reported, in a two-page item in Current Biology, that 63.8% slept with their left side down — a population-level bias, they argued, that lines up with the right hemisphere's job of watching for threats through the left eye [1]. Eight months later, in a paper posted online in February 2026, one researcher working alone, Andrew Cooke of the University of Lincoln, ran the same question on three independent platforms and found nothing distinguishable from a coin flip anywhere: 51.2% left among 123 YouTube cats, 48.4% among 494 on Reddit, 48.8% average among 60 Instagram accounts [3]. Sitting between the two papers, easy to miss, is a formal correction: in October 2025 the original authors published an erratum removing 107 videos with data-quality problems, prompted, Cooke says, by concerns he had raised — and the corrected number came out exactly where it started [2]. This field note is about the pair, not either paper alone. It walks through what a “population-level bias” is actually claiming, why that is not the same statistical object as an individual “preference” the way pawedness is, how three sampling frames disagree with each other more informatively than any one of them disagrees with chance, and why a claim, a self-correction and a failed replication landing inside eleven months is what a functioning literature looks like doing its job. Every comparison across the two papers below — the pooled sample sizes, the percentage-point gaps, the chance expectations — is the club's own arithmetic on their published numbers, marked inline as ours.

Two Papers, Eleven Months Apart

Most of the papers this club writes up sit alone. One team, one dataset, one set of conclusions that either hold up under scrutiny or don't. This one is different on purpose, because the interesting object isn't either paper by itself — it's the eleven months between them, and what happened inside that gap.

Paper one says two out of three cats sleep on their left side, and offers a tidy evolutionary reason why. Paper two, run independently on three different sources of cat photographs and videos, cannot find that bias anywhere. In between, the first team quietly fixed a data problem in their own dataset and got the same answer anyway. None of that makes either paper wrong in some scandalous sense. It makes them a case study in something the club has circled before, in pieces on what a p value can and can't tell you and on how to actually read a paper: a single result, however clean, is a claim waiting for someone else to try it a different way.

What the First Paper Actually Measured

Start with the claim as it was actually made, not as a headline would flatten it. Isparta and twelve colleagues — a group spanning labs in Turkey, Germany, Italy, Canada and Switzerland — pulled publicly posted YouTube videos showing a cat asleep on its side, full body visible from head to hind legs, holding that position for at least ten seconds [1]. After screening, 301 videos of 301 different cats went into the final tally. In 192 of them the cat lay with its left shoulder down; in 109, its right. That's 63.8% left, 36.2% right, tested against an even 50:50 split with a chi-square goodness-of-fit test: χ² = 22.9, df = 1, p < 0.001 [1]. Not a subtle result, statistically speaking — a departure that size at that sample size is not what a fair coin does.

Figure 1. The headline result exactly as published: 301 videos, one per cat, coded once each for which side was down [1]. Every cat in this figure contributes a single L or R — a detail that matters more than it looks like it should, and is the subject of §8 below.

The Hemisphere Story, and the Caveat Buried in It

A number alone isn't a paper; a paper needs a reason the number should exist. Isparta and colleagues reach for the same logic that has powered decades of lateralization research in other species: the vertebrate brain is not symmetric in what its two halves are good at, and in several species the right hemisphere specializes in processing threats, which gives an animal's left eye and left visual field a head start on spotting one [1]. Sleep on your left side, and your left eye faces outward into the room instead of into the floor. The authors frame the leftward bias as the sleeping-posture version of that same asymmetry — not a new mechanism, but a plausible use for an old one.

It's a clean story, and the paper is careful enough to test the most obvious version of it against itself. Cats are known to have individual paw preferences the way people have handedness: a meta-analysis of the existing literature put the fraction of cats with a measurable left- or right-paw preference at about 78% [4], and Isparta is a co-author on both that meta-analysis and a 2024 methodology paper on how paw preference should be measured [4, 5]. If sleeping side and paw preference share a cause, left-pawed cats should skew toward one sleeping side and right-pawed cats toward the other. The paper checks this reasoning explicitly, and rejects its own tidy version of it:

“Pawedness could be a further factor that affects sleeping position. Indeed, about 78% of cats show either left- or right-sided paw preferences. But since pawedness in cats displays an individual asymmetry with about equal numbers of left- and right-pawed cats, our population asymmetry in sleeping position of 65% is difficult to explain by paw preferences.” [1]

Read that quote twice, because it is doing more work than a single sentence in a two-page paper usually gets to do. Paw preference in cats is individually lateralized — most cats favor one paw or the other — but the population as a whole splits roughly 50:50 between lefties and righties, the meta-analysis found [4]. There is no population-level pull in one direction. Sleeping side, as this paper reports it, is the opposite shape: a strong population-level pull toward the left, cutting across individual cats. The authors' own honesty here previews the argument this whole field note is really about. A population bias and an individual trait are not the same statistical animal, and conflating them is exactly the crack the second paper gets a crowbar into.

An Erratum Arrived Before Any Rebuttal Did

Before anyone published a word disputing the result, the original authors corrected it themselves. In October 2025, Current Biology ran a formal erratum to the June paper [2]. The correction notice lists specific problems the authors found in their own 301-video-plus dataset: a handful of videos that had been marked as both included and excluded by mistake, several duplicate videos of the same cat counted more than once, some clips shorter than the paper's own ten-second minimum, one still image miscounted as a video, and a set of clips where the cat was sleeping on a person rather than alone. Fixing all of it meant removing a combined 107 videos [2].

Andrew Cooke, the author of the paper that follows in this field note, states plainly that he is the reason the correction exists: “Corrections to the work of Isparta et al. (2025) were published in October 2025 following concerns that I had raised” [3]. That is worth sitting with for a moment, because it means the same person who eventually reported a null result had already, months earlier, pushed hard enough on the original dataset to get a journal correction issued — before his own recheck was anywhere near published. Nothing about this sequence is unusual for how science is supposed to work. It is, however, easy for a reader who only saw the June headline to have missed entirely.

And the correction changed nothing about the headline number. After all 107 problem videos came out, the corrected dataset held at 301 cats, 192 left, 109 right — the exact same 63.8%, the exact same χ² = 22.9, p < 0.001 [2]. The errors were real. The conclusion, on this one dataset, survived them.

The claim, as it stood after its own correction
Quantity	Before the erratum	After the erratum
Videos analysed	previous total, including flawed clips	301
Left-side down	—	192 (63.8%)
Right-side down	—	109 (36.2%)
Test statistic	—	χ² = 22.9, df = 1, p < 0.001
Videos removed	107, across five documented error categories [2]

One Researcher, Three Platforms

Cooke's recheck, published online in Laterality in February 2026 and in print that May, doesn't reanalyse Isparta's videos. It goes looking for the same effect independently, in three places at once, with the inclusion criteria written down in enough detail that someone else could rerun them: a cat counts as leftward if its left shoulder sits outside its right shoulder and the curve of its body reads clockwise; a cat in a neutral or twisted pose doesn't count at all; two raters coding the same footage agreed on the classification 99.5% of the time [3].

YouTube. Rather than browsing the site the way a person would, Cooke queried the YouTube API directly — specifically to avoid the recommendation-algorithm and watch-history bias that can creep into a manual search — and screened 2,348 candidate videos down to 123 unique cats [3].
Reddit. Images from multiple cat-focused subreddits, 494 unique cats in total, with a 186-cat subset drawn from the single subreddit r/Catbun reported separately [3].
Instagram. 60 cat accounts, averaging 29 sleeping-position images per account — the one dataset in the paper built to let the same individual cat be checked more than once [3].

Three platforms, three different selection pressures on what gets posted, one shared question. If a leftward bias this size were really there in the world's sleeping cats, it should survive being looked for in a different place.

The Numbers, Side by Side

It didn't. None of the three datasets came anywhere near the original's departure from chance. YouTube: 63 left, 60 right out of 123, 51.2% left, χ² = 0.074, p = 0.786. Reddit, all subreddits pooled: 239 left, 255 right of 494, 48.4% left, χ² = 0.522, p = 0.470. Reddit, r/Catbun alone: 87 left, 99 right of 186, 46.8% left, χ² = 0.774, p = 0.379. Instagram, averaged across the 60 accounts: 48.8% left, SD 11.2 percentage points, t = 0.836, p = 0.407 [3]. Every single test lands comfortably inside what a coin flip produces.

Figure 2. All five reported samples, one dot each [1, 3]. One study sits 13.8 percentage points from chance with p < 0.001; the other four sit within 3.2 points of it, none significant. The gap between the warm dot and the four cool ones is the entire disagreement this field note is about.

Put a number on that gap, because eyeballing a chart is not the same as measuring one. Isparta's estimate sits 13.8 percentage points above 50%; the largest departure from 50% in any of Cooke's three independent samples is the r/Catbun subset, at 3.2 points — less than a quarter of the original gap (3.2 ÷ 13.8 ≈ 23%, our own division) [1, 3]. And Cooke's recheck, across the three non-overlapping samples, coded 677 cats and accounts in total — more than double Isparta's 301 (677 ÷ 301 ≈ 2.25×, again our arithmetic, treating the r/Catbun figure as a subset of the Reddit total rather than adding it separately) [1, 3]. This isn't four studies each squeaking past significance in different directions. It's one outlier and a wall of agreement around zero.

It's also not the first time this exact question came up empty. An earlier survey of spontaneous feline behaviour by McDowell, Wells and Hepper had already looked for lateralization in cat sleeping position in 2018 and found none [6]. Cooke reads his own three-platform null as lining up with that earlier result rather than overturning some settled consensus — it's Isparta's 2025 finding that was the outlier against the prior literature, not Cooke's recheck of it.

Even the Outliers Don't Point the Same Way

The Instagram dataset is the one place Cooke could ask a sharper question than “what's the population average,” because it's the one dataset with repeat photographs of the same 60 accounts. Running a z-score check account by account, Cooke found that only 2 of the 60 accounts were individually left-lateralized by a strict test, and 4 of the 60 were individually right-lateralized [3]. Six outliers out of sixty, splitting almost evenly across both directions rather than clustering leftward.

INSTAGRAM, 60 ACCOUNTS: SCHEMATIC, BUILT FROM THE REPORTED MEAN AND SD

Figure 3. An illustrative curve built from the paper's reported mean (48.8%) and standard deviation (11.2 points) [3], not a plot of the sixty individual data points, which the paper doesn't publish. Sixty accounts is roughly enough that even a fair coin flipped 29 times per account, with no bias in the world at all, would still be expected to throw a handful of individually “significant” accounts by chance — and here the six that turned up split 2 against 4, not lopsidedly leftward the way a real population bias would predict.

What “Preference” Is Actually Supposed to Mean

Here is Cooke's sharpest methodological point, and it isn't really about sample size at all. It's about what a single photograph of a sleeping cat can and cannot tell you:

“The experimental design has no capacity for the measurement of individual preference, which typically relies on multiple observations per individual.” [3]

Think about what “preference” means for a paw. Cats aren't tested once; a lateralization study watches an individual cat reach for food dozens of times and asks whether it reaches with the same paw consistently enough to call that a trait of that cat [4, 5]. A single reach tells you nothing about the animal — it could be a coin flip that happened to come up left that once. Isparta's videos, by contrast, capture each cat exactly once. Cooke makes the point directly: a population landing at 63.8% left is compatible with several very different underlying realities — “63.8% of cats sleeping leftwards every time, all cats sleeping leftwards 63.8% of the time, or a variation across the population that averages out at 63.8%” [3] — and a one-shot video cannot tell those apart. It can only tell you where the coin landed once, for each of 301 different coins.

WHAT ONE PHOTO CAN AND CANNOT ESTABLISH; SCHEMATIC, NOT MEASURED DATA

Figure 4. An illustration of Cooke's design point [3], not a graph of real cats. Isparta's videos give one letter per cat; only the Instagram arm of Cooke's recheck, with repeated images per account, can even attempt the right-hand kind of test — which is exactly the arm that found six mixed-direction outliers among sixty accounts rather than a leftward-skewed population.

Notes From the Club Table

First disagreement: Somebody argued that four non-significant results don't disprove one significant one — absence of evidence, evidence of absence, the usual warning. Fair, as a general rule. It matters less here because Cooke isn't reporting four weak nulls that might still average out to something real; the four estimates cluster tightly around 50% (46.8 to 51.2%) rather than scattering around some other shared number like 55 or 58. If a real effect were hiding in the noise, the four independent point estimates ought to at least lean the same direction. They don't consistently.
Second disagreement, not fully resolved: Isparta's videos and Cooke's YouTube arm both come from the same platform, coded with compatible definitions, and still landed 12.6 percentage points apart (63.8 vs. 51.2, our subtraction). Sampling method has to be doing real work here. Isparta doesn't publish exactly how the original videos were found; Cooke used the YouTube API deliberately to route around personal watch-history bias. One plausible story: a manual search, run from a logged-in account, tends to surface videos the algorithm already thinks that searcher will like, which is a channel a true population estimate should not have to pass through. We couldn't fully settle whether that's the whole explanation, and neither, to be fair, can Cooke's paper.
What we kept coming back to: The 78% pawedness figure and the 63.8% sleeping-side figure sound like the same kind of number and are not. One is the fraction of individuals with any stable lateral preference, split roughly evenly between left- and right-handed cats [4]. The other, as originally reported, is a population-wide pull in one specific direction from a single snapshot per animal. Cooke's whole recheck is, in a sense, an argument that the second number was never entitled to be compared to the first.
Standing caution: We are a school club with no lab and no cats to film. Everything in this piece is arithmetic on two published papers, clearly marked wherever it's ours rather than theirs.

Why an Eleven-Month Turnaround Is the Good Outcome

Line the dates up and the story reads less like a retraction and more like a functioning process. The original claim went online 23 June 2025. The authors' own correction went online 4 October 2025 — about fifteen weeks later, and, as noted, prompted by the same person who would go on to publish the recheck. Cooke's independent three-platform paper went online 24 February 2026 and reached print that May [1–3]. Start to finish, claim to correction to failed replication in print, the whole arc spans roughly ten and a half months — comfortably inside a year.

≈10.5 months, claim to a printed recheck (our count of the calendar)

Figure 5. Dates as published in the two journals' own records [1, 2, 3]; the elapsed-time count is ours. Note that the correction predates the recheck by more than four months — the original team fixed what they could find in their own data before an outside check of the finding itself had even appeared in print.

Set this against the backdrop the club has read about before: a 2015 project that tried to redo 100 published psychology findings and could only reproduce about a third to a half of them, depending on how you count “reproduce” [7]. That project took years, involved more than 270 researchers, and became a fixture of every conversation about a so-called replication crisis. What happened with these two cat papers is the crisis's inverse, on a much smaller stage: a specific, falsifiable, numeric claim; a self-correction of the underlying data within four months; an independent, multi-platform, methodologically transparent recheck within eleven. Not every finding gets that kind of attention that fast — sleeping cats are, frankly, a more shareable dataset than most psychology stimuli — but the process, when it runs, is meant to look exactly like this.

What Would Actually Settle This

Not another single-photo dataset, however large. The one thing missing from every arm of both papers is a design built from the start to catch individual consistency: the same cats, photographed or filmed sleeping many times over weeks, with a per-cat leftward proportion computed the same way pawedness studies already compute a per-cat paw-preference index [4, 5]. Cooke's Instagram arm gestures at this and found nothing, but 60 accounts averaging 29 images is a modest start for a question this specific, and Instagram accounts are not a random sample of cats to begin with. A registered, repeated-observation study — ideally preregistered, so the analysis plan is fixed before the data come in — would answer the question the two existing papers, for different reasons, cannot: not just whether cats as a group lean left, but whether any individual cat reliably does.

What We'd Tell Someone Who Only Read the Headline

Three things. The original 63.8% number was real, measured carefully enough that its own authors could find and fix flaws in the underlying data without the headline moving. It was also, on the evidence gathered since, an outlier: three independent samples covering more than twice as many cats found nothing like it. And the two facts that made this worth a field note aren't really about cats at all — they're that the people closest to a surprising finding corrected their own data before anyone forced them to, and that an outside check arrived on a timescale most fields would envy. That's not a story about a mistake. It's a story about a claim doing exactly what a claim is supposed to do: sit still long enough for somebody else to check it.

References

Isparta, S., Ocklenburg, S., Siniscalchi, M., Goursot, C., Ryan, C. L., Doucette, T. A., Reinhardt, P. R., Gosse, R., Çıldır, Ö. Ş., d'Ingeo, S., Freund, N., Güntürkün, O. & Demirbas, Y. S. (2025). Lateralized sleeping positions in domestic cats. Current Biology 35(12), R597–R598. doi:10.1016/j.cub.2025.04.043
Isparta, S., Ocklenburg, S., Siniscalchi, M., Goursot, C., Ryan, C. L., Doucette, T. A., Reinhardt, P. R., Gosse, R., Çıldır, Ö. Ş., d'Ingeo, S., Freund, N., Güntürkün, O. & Demirbas, Y. S. (2025). Lateralized sleeping positions in domestic cats [Corrigendum]. Current Biology 35(21), 5392–5393. doi:10.1016/j.cub.2025.09.068
Cooke, A. S. (2026). No evidence for lateralized bias or preference in the sleeping positions of domestic cats. Laterality: Asymmetries of Body, Brain and Cognition 31(3), 306–323. doi:10.1080/1357650X.2026.2633997
Ocklenburg, S., Isparta, S., Peterburs, J. & Papadatou-Pastou, M. (2019). Paw preferences in cats and dogs: Meta-analysis. Laterality: Asymmetries of Body, Brain and Cognition 24(6), 647–677. doi:10.1080/1357650X.2019.1578228
Isparta, S., Töre-Yargın, G., Wagner, S. C., Mundorf, A., Cinar Kul, B., Da Graça Pereira, G., Güntürkün, O., Ocklenburg, S., Freund, N. & Demirbas, Y. S. (2024). Measuring paw preferences in dogs, cats and rats: Design requirements and innovations in methodology. Laterality: Asymmetries of Body, Brain and Cognition 29(3), 246–282. doi:10.1080/1357650X.2024.2341459
McDowell, L. J., Wells, D. L. & Hepper, P. G. (2018). Lateralization of spontaneous behaviours in the domestic cat, Felis silvestris. Animal Behaviour 135, 37–43. doi:10.1016/j.anbehav.2017.11.002
Open Science Collaboration (2015). Estimating the reproducibility of psychological science. Science 349(6251), aac4716. doi:10.1126/science.aac4716