For as long as people have kept dogs, they have traded confident theories about boys versus girls. Females are tidier. Males are softer. Females mature faster. Males are the dramatic ones. Until recently, every one of those claims rested on the same evidence: a hunch and a sample size of one. Then the largest behavioral dataset in the history of the species arrived, and it did something the old kennel wisdom never could. It put numbers on the question.
The headline is not that the stereotypes were confirmed or busted. It is that the real differences are measurable and tiny at the same time, and that a factor most people never ask about, whether a dog is spayed or neutered, moves the needle more than sex does. Once you see the numbers, the whole “boy dog versus girl dog” debate looks less like a rule and more like background noise.
Where the numbers come from
The data is from the Dog Aging Project, a long-running community-science effort in which tens of thousands of owners fill out a detailed annual survey about their dogs. The behavior questions come from the Canine Behavioral Assessment and Research Questionnaire, or C-BARQ, a validated instrument that asks owners to rate their dog on a five-point scale across more than a dozen behavioral domains.
The sex-differences analysis, published in PLOS One in September 2025, drew on 47,444 dogs enrolled between 2020 and 2023, of every breed, age, and size. That scale is the whole point. A neighbor’s opinion is an anecdote; 47,000 dogs sorted into consistent categories is something you can actually measure against.
Researchers grouped the responses into four behavioral buckets to compare the sexes: fear, attention and excitability, aggression, and trainability.
The four differences, smallest gaps and all
The data found a real difference in each of the four areas. What matters is the size, so the numbers are worth keeping:
Males were rated less fearful. Male dogs scored about 0.106 points lower than females on fear-related questions. Whether that is genuine boldness or simply less caution is anyone’s guess.
Males were rated slightly more aggressive. This was the narrowest gap of the four. Males averaged roughly 0.051 points higher on aggression directed at other dogs, strangers, and their own households.
Females were rated more trainable. They came in about 0.106 points higher than males on trainability.
Females demanded less attention. The widest gap of the set showed up here. On attention-seeking behavior, the cluster that covers chasing, attachment, and separation-related behavior, females scored about 0.135 points lower, meaning less need for attention.
Sit with those magnitudes for a second. The biggest of the four differences is roughly a seventh of a point on a five-point scale. These are faint averages across a stadium of dogs, not traits you’ll reliably spot in the two dogs at your feet.
The finding the old debate never had: the spay/neuter effect
Here is the genuinely new contribution, and the part worth bringing home from the study. When researchers looked at reproductive status, the differences they found were in the same league as the sex differences, sometimes larger.
On average, spayed and neutered dogs were rated more fearful, more aggressive, and less trainable than intact dogs in this dataset. That cuts directly against the folk assumption that “fixing” a dog automatically mellows it.
“This is exactly the kind of result that needs careful handling,” says Dr. Lena Park. “It is owner-reported, observational data, so it shows an association, not a cause. People often spay or neuter precisely because a dog is anxious or reactive, which can make altered dogs look more fearful in the numbers without the surgery being the reason. Nothing here changes the strong medical and population-control case for spaying and neutering. What it does is retire the lazy promise that the procedure is a behavior fix.”
That nuance is the takeaway: reproductive status is woven into a dog’s behavior profile at least as tightly as its sex, and neither one works the way the bumper-sticker version claims.
A second new wrinkle: trainability is drifting
The same body of Dog Aging Project research surfaced something the boy-versus-girl framing completely misses. Across enrollment years, average reported trainability declined, with dogs who joined in 2021 through 2023 rated lower on trainability than the 2020 cohort, even as fear, playfulness, and aggression held roughly steady.
No one is certain why. It overlaps with the wave of pandemic-era adoptions and the disrupted socialization windows that came with them, which is a plausible but unproven explanation. Either way, it is a reminder that when and how a dog grows up may shape behavior more than which sex it happens to be.
The rest of the breakdown
Sex was only one lens. The researchers also sliced the data by breed, size, age, and region, and a few results are worth keeping:
- Small dogs really were rated scrappier, scoring higher on aggression and a touch lower on trainability than big dogs.
- Mixed-breed dogs scored high on attention-seeking.
- Dogs working as service animals were rated less aggressive and more trainable, which tracks with how they are selected and raised.
- Regionally, Northeast dogs were rated less aggressive than Midwest dogs, while Midwest dogs were judged more trainable, the kind of finding that says as much about owners as about dogs.
The lead author flagged the obvious confounder on the small-dog results: tiny dogs are simply easier to scoop up and move, so undesirable behavior often gets managed rather than trained, which can quietly depress their trainability scores without reflecting any real difference in capacity to learn.
So what do you actually do with this?
Treat all of it as gentle background, not a buying guide. Every number here is a group average pulled from owner perceptions, which means human expectations are baked in too. Researchers note that gendered assumptions about personality can leak into how people rate their own dogs, which is its own source of bias.
If you are choosing a dog, the honest advice is the boring advice: meet the individual, ask about temperament and history, and plan to socialize and train whatever you bring home. There is no shortage of fearless females, unbothered Chihuahuas, and gloriously independent mutts walking around to prove the averages don’t decide anything for you.
