President Barack Obama has visited the cell on Robben Island where former South African president Nelson Mandela spent 18 years of his 27-year imprisonment.
The visit comes 15 years after Bill Clinton visited the cell with Nelson Mandela himself.
More top news
The study looked at the development of 422 children over the course of eight years.
The president said they will come into force if China refuses to change its practices.
Laura Bush called the policy “cruel” and “immoral”.