When we examined whether regular help with homework had a positive impact on children’s academic performance, we were quite startled by what we found. Regardless of a family’s social class, racial or ethnic background, or a child’s grade level, consistent homework help almost never improved test scores or grades. Most parents appear to be ineffective at helping their children with homework. Even more surprising to us was that when parents regularly helped with homework, kids usually performed worse.

I can't say I'm surprised. At the high school level, many of my students regularly receive help from their parents, and the results are frequently negative:

Parents complain that the work is too hard or the assignments are unclear. Since they have never attended my class or received my feedback, I can see why they think so! But, likewise, I find this kind of criticism unfair. I would much rather hear from my students.
The corrections parents make to their children's work are often incorrect. In particular, they have a predilection for the passive voice (e.g., "Edgar Allan Poe is known for his impressive writing,") which I have made it my mission to eradicate in student writing.
If students assume their parents will be helping them with their assignments, they will put forth less effort in communication, time management and locating resources -- the three main components of a successful homework assignment.

The article goes on to say that parents can be great motivators, and that they should go out of their way to communicate the value of education to their children -- insisting they keep their grades up, limiting leisure and extracurricular activities during the school year, and choosing schools where their children will be able to succeed with hard work and determination. But this helicopter parenting, in which parents are constantly communicating with teachers about their nearly-adult children, is detrimental to all three parties -- children, parents and teachers.

As Robinson and Harris conclude, "What should parents do? They should set the stage and then leave it."

Amen!