11/08/2026
The short answer is we don't know for certain, but the evidence suggests that castration can increase fear and anxiety in some dogs, while having little effect or even improving behaviour in others.
Here's what we know.
1. Testosterone doesn't just influence reproduction. It also acts on areas of the brain involved in emotion. Some testosterone is converted into estradiol and some into neurosteroids that help regulate GABA, the brain's main inhibitory neurotransmitter. These systems help dampen fear and improve resilience to stress. When testosterone is suddenly removed, some dogs may become more sensitive to novel situations, more reactive to startling stimuli and slower to recover after a frightening event.
2 Testosterone appears to increase confidence, exploratory behaviour, persistence and willingness to approach uncertainty. After neutering, some already sensitive dogs seem to become less willing to investigate unfamiliar situations and instead choose avoidance or defensive behaviours.
3. Many behaviours owners describe as "aggression" are actually driven by fear.
If neutering increases fear in a susceptible dog it may simply be a consequence of heightened anxiety.
4. Most studies show enormous variation. Some dogs improve, show no behavioural change or become more fearful. Genetics, age at neutering, breed, previous learning experiences, pain, and environmental factors all influence the outcome.
What does the research show?
McGreevy et al. (2018) – found increased odds of fearfulness and some forms of aggression after neutering in certain populations.
Farhoody et al. (2018) – found no overall reduction in aggression after gonadectomy and reported increases in some fear related behaviours.
Zink et al. (2014) – discussed behavioural and health consequences of early neutering, particularly in working and sporting breeds.
Spain et al. (2004) and other studies have found mixed effects, highlighting that outcomes vary considerably between dogs.
If a dog already has a tendency toward fear, it's important to weigh the potential behavioural risks alongside the medical and population control benefits.