Single Parent vs. Both Parents
Is a child more likely to brought up correctly with both parents than with one parent?
I don't think you can judge that fairly. Maybe the child has both parents home but only one of them is truly a parent. The other one might not be there, a drunk or even abusive. There might be a greater likihood that a child's physical needs might be met with both parents, but not necessarily emotionally. I think its a case by case basis for sure.
Yes this has to ba case by case judgement. Who says that a single parent can not do just as well raising their child as the married parent whos partner is never home to help out? I believe that looking at this as a whole one can never truly make a general statement that will fit this mold.