In the West, it is believed that having multiple children is a basic right. Overpopulation, however, is the root of scarcity. Pollution, starvation, water shortages, the rapid depletion of energy sources such as oil, and the rampant over-consumption of physical materials and resources (these are just a few examples) are all symptoms of overpopulation. Reducing the population would remedy most of the challenges associated with scarcity. In my opinion, China's one child policy should be expanded to encompass all the of nations of the Earth.
For this particular country, being the epitome of overpopulation, there should definitely be limitations! We are smart enough to think of the possible consequences, as we should be in taking action for prevention. If you disagree still, I ask you: would you leave a tumour to spread? Of Course not, you would have it removed right away. We can't just let it sit there and wait until its too late. One problem is our attitude towards the idea being misunderstood by the opposition. We are not selfish in that we want this for our convenience, rather, we are concerned for the fellow human beings of the future! I respect all arguments against, as this is a very delicate topic, but I firmly agree it to be necessary!
Throughout the entire world the people who have the highest number of children are the uneducated, the poor, and if applicable those who can use the hands (such as farmers) which in turn leaves China as a prime contender for this particular problem due to it's size, it's culture, and it's expansive steppes which hold large farms.
Sadly an unrelated issue dealing with the preference of males to females, which too is cultural, has caused extreme hardship which was not foreseen. As a side-note India did not fare any better and is in worse ethical shape (http://www.Newslaundry.Com/2014/01/06/indias-one-child-policy/)
One child by law has created many problems for the country such as a unproportional male-female-ratio and killing of newborns.
For a general survival of the species there are at least two children required from each couple. Considering how many people in western countries choose to stay without children, it's even more.
The problematic countries are Africa, India and some Asian countries where either contraception is not common or unavailable. But they don't need a law that encourages cruelty against newborn (especially when they can't prevent getting pregnant due to contraception problems it's absurd to outlaw getting pregnant).
Overpopulation can not ethically be defeated with a one-child-law. India has shown successful approaches without forcing a ridiculous law upon it's people.