Why are Some Dogs Afraid of Men

Welcome back to Science Sunday

DueNorth's Puppy Playpen

Boarding clients dropping their dog off often tell us that "he doesn't like men", explaining they believe he was abused by a man in a previous home.  But I always wonder if the dog is just afraid of strangers in general without regard to gender.
Can dogs even discriminate between human genders? A 2014 study devised an experiment to determine just that. Fifty-one dogs were played a prerecorded male or female voice in the presence of a man and a woman. The responses were scored as correct or incorrect from both the direction of the first look and the total gaze duration towards each person after the voice presentation. The interesting element from this study was that dogs raised in a single person home identified male or female incorrectly 71% of the time, while those in homes with at least one male and one female were correct 80% of the time. So that it seems that dogs raised only by females may not understand what a male human is. 
Anecdotally, in our experience people reporting this problem generally have rescue dogs often with unknown background. A 2009 study in the state of Michigan found that 92% of dog rescue organizations are staffed by female volunteers. This preponderance of women may exacerbate the problem of the dog not being able to identify a male.
But why should men be so scary. It may come down to the Ying and Yang of gender difference. Men, generally, are bigger, louder, more assertive and forward than women. In extreme cases even people can identify when an assertive male walks into a room, its' reasonable to assume that dogs will clue into this in even less extreme cases.
So the solution, as with many canine behaviour problems is socialization. Pups need to be gently introduced to every type of situation possible, especially during their "fear periods".
Dogs go through two periods when bad experiences can be imprinted for life. The first is actually two periods, the first at five weeks, when  pups demonstrate a strong fear response toward loud noises and novel stimuli, then again at seven to twelve weeks, the puppy is very sensitive to traumatic experiences, and a single scary event may be enough to traumatize the puppy and have life-long effects on his future behaviours. In our own breeding program we begin to introduce novel odours just a few days after birth, and in the puppy pen present every possible type of surface and toy.
The second fear period occurs between six to fourteen months. In the wild, dogs at this age are allowed to go on hunts with the rest of the pack, and it may be a survival strategy for them to learn to respond fearfully  to the unfamiliar.
There are undoubtedly cases where a fear of men was caused by abuse, but it is more likely that fear of men, or anything else was a result of incomplete socialization at critical periods in the young dog's life. 

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References:	Why Some Dogs Are Afraid of Men, Dog Psychology 101, Feb 3, 2018
	Understanding Fear Periods in Dogs, ​ PetHelpful, Dec 31, 2019
	Why Dogs are More Afraid of Men, The Other End of The Leash, Jan 26, 2010
	Cross-modal discrimination of human gender by domestic dogs, Science Direct, May 2014
	​Women and the World of Dog Rescue:A Case Study of the State of Michigan, ​University of Michigan, 2009