This is interesting, and kind of tough to answer.
I'll give my experiences with my own two girls and the Doxies I've known:
Lupi-We got her at 4 weeks old as a rescue (in Mexico) and have been working non-stop to socialize her ever since. Her nature is quite passive. She didn't start growling at people or barking territorially until she was about a year. In fact, the growling was more like 2 years. She would constantly roll on her back when she met another dog. Now, looking back I realize she was scared of people but at the time I thought she was just "submissive". I wish I had made her socialization more positive. Anyway, once she was fully adult, she starting showing fear-aggression towards strangers, especially men. By aggression I mean she barks defensively or growls if they try to pet her. She also barks when people come to the house, but that just being territorial. Lupi is 6.5 years old and has never nipped anyone, not even the vet when she was in severe pain or kids who've kissed her on the muzzle. We've worked hard with her and she is much more accepting of strangers than she was in the past, even approaching them on walks at times. With friends and family she is a total love and I trust her completely with my nieces and nephews. With strange dogs, she's fine unless they're a large-breed puppy trying to play. Then she tells them off. So although she's not social with strangers, I would definitely not call her aggressive. Like Nell's Connie, she is also very smart and completely obedience-trained. So it's easy to distract her from her triggers and most people wouldn't realize she was a fearful dog if they met us on the street.
Lacy-She's from a good breeder but was extremely shy from the get-go. The noisiest, most anxious puppy of her litter. Possibly the most timid puppy I've ever seen. But also very smart. We got her at 11 weeks. She was raised with other dogs (including other breeds), cats, kids etc. Her parents were both champions and she had lots of time with her Mom and litter mates. The opposite of Lupi, basically. Yet she is more fearful than Lupi ever was. She's been barking and growling at strangers since the day we got her! She practically lived under our furniture for the first few weeks and housetraining was a nightmare since the sound of a leaf blowing would send her running back inside the house. We immediately started socializing her. She went to a puppy daycare program for the first couple of months, where they train and socialize the puppies using positive methods. That was 3 days a week. Plus we took her to puppy classes and on outings to pet stores etc on the days she wasn't in "school". Now she attends regular daycare, twice a week at the same facility. Almost every day, Lacy meets at least one new person. She is now 5 months old and has gained a lot of confidence. She will play in the yard by herself sometimes, and warms up to people much more quickly. Walks are still a huge challenge. Obviously, she's too young to be truly aggressive, but this is a dog with the potential to be a fear-biter. Only time will tell if nurture can overcome her inherent fearful personality. Like Lupi, Lacy is completely sweet with friends and family, but she is still very young.
Now our friends have a Doxie from unknown origin, completely untrained, who thinks every person she meets is her best friend. She is not the brightest girl, but honestly, it's the smart ones who seem to have more issues. We rescued a male Doxie a few years back who knew absolutely nothing when we got him and learned all kinds of tricks and obedience commands in the couple of months we had him. He figured out how to open drawers on his own, and could use them as stairs! He was awfully fear-aggressive. He bit a few family members quite severely, and we decided we couldn't keep him since our home is so busy, he would have ended up spending most of his life in isolation. He was also aggressive towards other dogs and would bite any dog he got close enough to reach.
Other friends of ours have Doxies who've nipped, but not bitten seriously, which still counts as aggressive to me. I know the breed standard says shyness is a fault, but I've met very few who weren't hand-shy with strangers. And they do tend to nip if their warning growls are ignored.
Overall, I have to agree that there is a tendency towards aggression in the breed. These dogs are awesome for their owners, but not always so well-liked by strangers. I always tell people that to truly appreciate a Dachshund, you have to live with one or at least get to know one well. They are so loyal and sweet once they trust you. With tons of work, they can be quite good with strangers too, but unless you're willing to do the work, I don't think it's fair to own a Dachshund.
