Are Neutered Female Dogs More Aggressive?

Updated November 21, 2023

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 The Unpublished Study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 https://saveourdogs.net/wp/wp-content/uploads/2010/10/Aggression-and-spay-neuter-in-dogs.pdf

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Summary: The big problem here is that this data is unpublished, and the presentation does not allow the reader to assess either the methods used for data collection, the analysis, or the results gained (like I did below).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. 

   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!

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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By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. Meet his team here.