A retrospective study looking at problematic behaviors and the age at which the puppy was taken from the litter, found that puppies taken earlier then 8 weeks displayed more behavioral problems that those who remained with the litter for the whole 8 weeks.
In addition, the authors found that puppies in pet stores exhibited more problematic behaviors than puppies from other sources even when they were kept with their litters for the 8 weeks!
Does this mean puppies should remain with their littermates for 8 weeks? It means that they should remain with their littermates for at least 8 weeks! More studies need to be done to really answer this questions but at this time it seems reasonable that no one should remove a puppy from his or her litter until at least 8 weeks and puppies should not be purchased at pet stores.