Well, it depends. According to various sources and my own observation, some female dogs become more aggressive, but some become less aggressive.
I guess you could call this normal.
There are four types of dog aggression:
Dominant-Aggression- This type of behavior is evident when your dog’s status within the pack (your household) or the community (other dogs and/or people) is threatened.
Fear Aggression: This type of behavior is evident when your dog become frightened and believes that his physical safety is at risk.
Protection Aggression- This type of behavior is evident when your dog believes that he must protect something or someone. This type of aggressive behavior may also be manifested as Territorial or Possession Aggression, when your dog displays aggressive behavior to prevent danger to someone or something that he controls or owns. Similar in nature, they vary primarily by the factual setting where they are occur.
Redirection Aggression- remember Giuseppe the neighbor who wanted to say hi in our first example? Well, suppose that another neighbor comes over and is unfriendly toward your dog. He might throw a rock at him. Your dog gets riled up, his anger starts to boil. The rock throwing neighbor hops the fence and leaves. Guess what might happen? Yup, Giuseppe might be on the receiving end of growling or in the worse case scenario, a bite.
And a website says that dog aggression can be SOLVED by spaying...
Spay/ Neuter- Dog Aggression is less evident in female dogs that have been spayed and male dogs that have been neutered. If you are making an appointment at the vet for a checkup, this might be a good time to get this safe and beneficial procedure performed.
but a website also says spaying makes female dogs MORE aggressive because the estrogen, which mediates the testosterone put out by the adrenal glands is absent.
so, it depends.
~hope you find my answer useful! if you do, vote it as the best answer! thanks in advance~