Are Neutered Female Dogs More Aggressive?

Updated November 29, 2020

What is the evidence for behaviour change in desexed female dogs? There’s a lot of discussion and debate, so let’s drill down into the data.

The Evidence For Aggression In Spayed Dogs

These are the only three studies I have found in a literature search that claim a significantly higher rate of aggression in desexed female dogs. Before I discuss these claims, I need to explain how scientists use the word ‘significantly’. It has a precise use, meaning that the odds of the result happening by chance alone are less that 1 in 20. It does not mean the effect is large or important, as you might think, just that it is probably a real finding.

Beware: I see writers quoting studies that do no more than showing more neutered females than intact females for a particular problem. This is meaningless without knowing if this differs from the percent of neutered females in the population at large.

1. The Unpublished Study

Most of the discussion about aggression in neutered female dogs is based on a Powerpoint presentation based on an online survey of over 7500 dogs. You can read it yourself at

  1. There are some powerful graphs in the slides and a very convincing argument is made for aggression being increased.
  2. If these findings are real then they will revolutionise our understanding of the effects of neutering.

Summary: The big problem here is that this data is unpublished, and the presentation does not allow the reader to assess either the methods used for data collection, the analysis, or the results gained (like I did below). I have contacted the author who told me its publication was delayed due to funding issues. Once it’s published, I’ll report on the findings here.  Read here about extracting information from peer-reviewed scientific papers.

2. The Edinburgh Study

O’Farrell & Peachey in 1990 reported a significantly increased risk of aggression in neutered female dogs versus intact bitches. After reading the original I need to highlight two important flaws in this paper:

  1. This risk was greatest in puppies under one year already showing some aggression, making the causal link questionable
  2. desexing study limitations
    Have a look at the differences between the two groups before the surgery was done (P values less than 0.05 are considered significant). I don’t think a study that had such unequal groups being compared would get published these days. All of these pre-existing factors could have increased aggression in the spayed group, especially being acquired at an older age (think about rescue dogs for example)

Summary: all we can say from this study is that more work is needed

 3. English Cocker Spaniels

A postal survey was conducted to investigate aggression in English Cocker Spaniels (reference at end). Neutered female dogs were found to be significantly more likely to show aggression towards children in the household. If you are reading this paper, make sure you read the follow-up study at the end where they factored in prior aggression.

Summary: this study appears to show a real effect

The Evidence Against Aggression In Spayed Dogs

As I said in How To Read Science, the best source of good information is a recent review of the scientific literature, which summarises all the relevant research up to that time.The most recent scientific review article on aggression and neutering is in early 2017 (reference below). I have a copy and am happy to share it on request. Here is the relevant excerpt from the conclusions:

The studies found that desexed dogs were associated with a reduced risk of dog bite, possibly in favour of reducing the risk of serious bite. However, differential effects for male and female dogs were not able to be examined in detail due to the data available but may possibly be greater for females than males

Summary: the authors suspect that desexing females actually decreases aggression more than it does in males; how confusing! Currently, there is insufficient evidence to say either way.

In conclusion, effects of desexing on behaviour in female dogs may exist, but they are likely to be small and possibly confined to specific breeds or situations. It’s also possible that if it exists, this aggression may be partly due to the process itself of desexing (ie. was handling gentle, was pain relief used?). Despite many studies to this date, there is only limited evidence and other factors such as puppy socialisation and training are likely to be much more important.

What about other harmful effects of desexing?

I have written a long, fully referenced article on the pros and cons of desexing. Please visit and leave a comment!

You may also like my views on Ovary Sparing Spay

D’Onise, K., Hazel, S., & Caraguel, C. (2017). Mandatory desexing of dogs: one step in the right direction to reduce the risk of dog bite? A systematic review. Injury Prevention.

O’Farrell, V., & Peachey, E. (1990). Behavioural effects of ovariohysterectomy on bitches. Journal of Small Animal Practice, 31(12), 595-598.

Podberscek, A. L., & Serpell, J. A. (1996). The English Cocker Spaniel: preliminary findings on aggressive behaviour. Applied Animal Behaviour Science, 47(1-2), 75-89.

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Have something to add? Comments are welcome below and will appear within 24 hours.
By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These articles are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story!


One Reply to “Are Neutered Female Dogs More Aggressive?”

  1. Dear Andrew,
    Thank you for these articles on desexing / spaying & the are female desexed dogs more aggressive.
    There is so much conflicting data / articles available on desexing and at what age & everyone interprets to their beliefs rather than actual medical evidence / advice.

    This article of yours has helped me greatly.
    Until recently I had always desexed at 6months of age, b4 bitches go through their 1st season as I do not want to breed nor want the added mess of a bleeding from her 1st or subsequent seasons.
    Recently had been persuaded by other dog owners to let my bitch fully matured to 1 year b4 desexing as it was ‘better’ 4 the dogs health.

    I have a 12 week old Maltese x Shih Tzu bitch. I know it’s said small dogs @ 6 months of age & med / large breeds 12-18 months but, I had read several articles warning me to wait till she fully matured (but am now wondering if that is only meant to refer too medium / Large dogs), but last week after being bitten by a spider 4 times (Once on her Valva & 3 next to it), l found a hernia on her upper abdomen / lower chest region after searching 4 bites (Will come back to hernia).
    The poor little thing was in agony for 3 days. I know exactly how she felt, I too was bitten by a spider the week b4 last.
    I could not go without an ice pack on my foot 24 / 7 for 7 days, even requiring to sleep with a damn thing on as every time I took off it would just burn and sting like mad.
    I have no idea what type of spider bit me, but I found a baby huntsmsn or something similar looking on Luna’s chest (Unfortunately could not identify the spider in photos on any Australian spider ID site.
    Luna had been biting / scratching like mad & yelping since the night b4 (she was only Flea/ H/W & Wormed on the 28th December, so I knew it wasn’t from fleas, but I could find no bites. Neither did the vet.
    Once home she attacked herself again & this time I found the 4 bites, taking her straight back 4 a collar, consultation & cream (the cream seemed to aggravate her more though and favoured a cold pack instead.
    At least this kept her still with applying the cream she would get up and run the length of the lounge room all the lounge every 5 to 10 seconds.
    I found it easier just to Cradle her in my arms like a baby & with her head on my heart it too calmed her as did the ice pack wrapped in a face washer compressed to her lower regions or at night I would bandage it to her so I could sleep too.

    Back to the Hernia…
    Talking about her hernia (it is very small in size, oblong in shape & 1cm in size.
    I know from a failed attempt at buying the last pup in september, that an umbilical cord hernia means fixing while desexing now at 5-6 months unless it grows or changes abnormally, then requires immeaditly attention while desexing.

    I am using the National Desexing Network scheme. Instead of looking at $400-$450 it will end up costing $155.00 which I’m quitely excited about.
    More money for training / outings (once fully vaccinated of course), treats & toys 4 us! She Love, Love, Loves gifts / presents & can’t wait to find out what’s inside (so much fun to watch)!
    She is already spoilled rotten an only child in a no children household!
    I have several injures / disabilities & she Luna is a therapy aid / companion animal.
    Due to my injuries & migraines, I have tried to cover all bases in case I go down days at a time with migraines (a frequent occurrence especially March – July) so purchased a 6 day automatic pet food bowl with LCD screen and voice message in my voice calling her 4 dinner.
    It has 6 compartments for food & treats which u fill each day.
    I also bought a 3.8 L automatic water bowl, an indoor potty which do far she lies on, and iFetch Frenzy machine that she can put balls in & sends them out over 3, 6 or 9 meters that she fetches and plays all over again, as well as the usual food balls, balls, chew rungs and ropes (no tug of war till adult teeth come thru, is that 6-9 months of age or is it older?) & so much more.
    We bought 2 DVD’S from UK Training experts Tom Mitchelle & Lauren Langman from Absolute Dogs called Boundary Games & Leash Off / Games On! They are fantastic!
    We use her Hills Science Diet dry puppy food (from her 1st feed of & at least 1 other lot of her Canin Dry Puppy food (arrived at the vet’s on tuesday, just as the Canin Rep arrived, think she converted me from Hills / Advance / Ivory Coat to Canin prescription Dental too, (amazing tech in them), I had the Hill’s Dental, but Kenzy my last Malshi was allergic (just 2 dental bites).
    So we use the dry to play / re-enforce all these training methods.
    Luna is super smart too!

    Unfortunately I lost my last therapy aid Kenzy at just 6.5 years on Aprill 11th 2017 to some sort of blood disease where she was destroying her red blood cells & not producing new one’s (& only then baby red blood cell after 2 blood transfussions, but much too late).
    I thought it kinder & best 4 her to bring her home for a day after a week of hospitalization and had our family vet come to thd house to put her to sleep.
    But boy did we both need that day Dogs that! We’re not used to being apart more than a few hours.
    She was attached at my hip, (my simease twin)!
    I needed to give her a second chance even though I knew it was unlikely to work, a 2nd blood transfusion.

    Believe it or not that day at home together was the best, most magical day of my life, even though it ended as one of the worst, but what occured overshadowed/ Topped even her passing!
    I won’t write the whole long sad story as it’s over a page long, But to summesummarise It was ‘The Perfect Day’ (apart from her troubles & struggling to breathe which started that morning), the sun was shining big white fluffy clouds, multiple bird types cockatoo, galahs, yellow self crested cockies, even a Willy wag tail (which we never get due to the nasty grey minor birds) Cabbage moths, monarch butterflies galore (never seen so many in one day b4), bugs / insects of all kinds chipping chirping chattering & making the noises insects make, just pure peace, bliss & joy!
    It was so peaceful & spiritual ethereal and best of all no car noises (which is unusual we’re on a 4 school exit route being only 1 of 2 exits / entries to all 4 schools).

    I’ve never experienced anything like it before in my life & I think it’s what enabled me to heal although of course I cry & did do, I still cry every now & then as I just did now, while writing this, but I knew it was what was right for her & was what she needed and that’s all that mattered.
    I held her for 3-4 hours which was killing my injured back / legs / shoulder but could let her go.
    Kenzie was a pure 100% inside dog and hated the outdoors but this day she only wanted to sit outside in the garden there was a slight breeze and I think it was probably made it easier for her to breathe too.
    But it certainly took me a long time to get over.
    Kenzy was a beloved therapy aid as well as my fur-child.

    Thanks once again for this article and debunking many myths & telling it like it is rather than scare mongering and putting profits a head of care.

    I have always been a great advocate of desexing our pets and being a responsible pet owner and take it very seriously.

    I also help run a Facebook group called Maltese x Shih Tzu Australia which is where I found this article today posted by our Admin Manager.

    It’s a shame I live in the Northern suburbs of Adelaide or else you would have had a new client today. Although I did travel too Walkerville 4 migraine physio so who knows, plus I’ve signed up 4 the pod casts / articles / Facebook group.
    Unfortunately too far 4 an dog emergency though.

    I will recommended u to any Eastern Suburb / City resident Friends / Family though.

    Is there anything I haven’t maybe thought of or you can think I may need to know about desexing my puppy. Or anything else I may need to know about anything in particular to breed / age etc?

    I know of knee / hip dysplasia (& have stairs once she grows a bit 2 use 4 our bed), the common eye conditions & possible skin conditions affecting this breed (Kenzy, had severe skin problems & hot spots / anxiety. Bought her a cool sports Thundershirt from the US. )

    I will not be breeding from Luna (I know hernia’s are a genetic fault breeders want out of the Gene pool and am annoyed the breeder failed too inform me this little one had a Hernia.

    My breeder was not forth coming with questions or answers to my Questions and I got more info re their food milk / types / quantities from Roxy’s (Luna’s little sister’s owner Jacqui), than I did through the breeder. Not right not when charging $900.
    I couldn’t even get photos or footage weekly once I paid my deposit or at all until I told her I refused too leave home & pay the remainder / pick her up until I saw a new photo as they change so much between the ages of 5 weeks & 10 weeks, when I got her & was worried it could all be a scam as breeder chose to ignore all questions I’d put at her.
    At least all the other buyers got to deal with The Breeders Mother who actually had the pups & answered questions but noone told me of this besides Jacqui!

    I never actually got to meet Luna before purchasing her, as I was meant to drive over to Nhill in Victoria to purchase her but, the night before leaving I was electrocuted and was very ill for 3 days and ended up having to send a Pet Courier Service to pick her up, but she had been looked over briefly by one of the ladies in our Facebook dog group as Jacqui bought Roxy Luna’s young sister!
    I appologies 4 thd length of my letter and hope it did not waste much of you’re valuable time.
    We look forward to reading more & watching too!
    Thanks Dr Andrew!

    With Kind Regards,

    Julie & Luna Pederick.
    ‍♀️ &

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