Do you have references for why the push for early academics is bad for children?

I have a friend with two young children.  He and his wife are Indian.  Their culture has a strong push for success in school.  Many have a strong push for early academics.  This means that even after a full day of school the parents will work with the children to do all the homework and then they'll even supplement with more academics.  Long hard hours in high school do have a positive correlation with increased mastery of the subject, but I've read that ten and twelve hour days before ages 12 have a decrease in long term comprehension.

And from a brain development stand point this makes total sense.  Children's brains don't finish developing until they are in their late teens.  We don't ask a two-year-old to do calculus, but public schools see no problem pushing hard to have five and seven year old children start down the academic road with full force and speed.

My friend was surprised when I shared that lots of academics for young children was bad in the long run.  He asked for references.  I found this one: Full-day kindergarteners' reading, math gains fade by 3rd grade. 

Do you know of any other references?

Thanks.