Is stimulus generalization acquired due to inability to discriminate stimuli? The classic case of Stimulus Generalization is Little Albert. The About.com link also explains it in terms of dogs. I note that in both cases, it seems highly likely that the subject (dog or infant) is unable to discriminate stimuli or unable to determine which part of the stimulus is relevant.

Have there been studies on stimulus generalization in adults? 

Specifically, I wonder if stimulus generalization results from the inability to determine what part of an unconditioned stimulus (white rat) is associated with the conditioned stimulus (loud noise). 

I know that discrimination can be caused by conditioning with relevant stimuli and extincting non-relevant stimuli, but would an adult human generalize fear to a general stimulus or a specific one and why?
If you think of the stimulus being represented by a distributed set of features (Tanifuji et al., 2001), then I would not say it was a failure to discriminate. During the conditioned response training for Albert, all features were present as the stimuli. It wasn't that he couldn't discriminate the features, just that each of the features were trained as conditioned stimuli.  

I suspect that an adult would think of the rat symbolically, and the conditioning would be done on that mental rat symbol. The individual features would likely not produce fear. Children of Albert's age are not thought to have yet developed symbolic thought (DeLoache, 1987). So, the conditioning would be done on the visual components themselves.

References

http://anon.cs.rochester.edu/users/faculty/dana/tanifuji.pdf
http://www.sciencemag.org/content/238/4833/1556.short