I agree with Susan. I have 2 boxers, both female, and the older girl, Tess, was constantly teased by a male neighbor behind a solid 6' fence. He would call her over as if to give her a treat, and then either spray her in the face with a hose or throw rocks at her.
This is an older man mind you, and the only thing Tess could see over the fence was his baseball cap. She was very young at the time this occurred but the impact is still with her (she's 7 now). Sorry for all the background - anyway Tess will growl and show her teeth anytime she encounters ANYONE wearing a ball cap or other type of hat, including my 14 year old daughter (Tess lunged at her). We tried an experiment with the cap - Nikki would put the cap on , Tess would growl, she'd take the cap off to show Tess it really was her Nikki, and then put it back on and the growling would start again. Consequently, no one is allowed to wear a hat of any type when they are in our home or in Tess's presence.
Good luck -
Jen - mom to Tess, 7 fawn female, natural ears & Tenaia, 2 dark brindle female, natural ears