The decrease was marked. • 90% decrease of these behaviors in 40% of the study dogs. • 50% decrease of the remaining 60% of the study dogs. No relationship existed between the effect of neutering and the age of the dog or duration of the problem behavior before castration. Neutering also affected aggression toward canine and human family members but to a lesser extent and in fewer dogs 25% of the study dogs improving by more than 50%.