In a world with rampant over-population, families should not be having more than two children. Birth rates need to drop below replacement.
http://en.wikipedia.org/wiki/Sub-replacement_fertility
Sub-replacement fertility is a total fertility rate (TFR) that (if sustained) leads to each new generation being less populous than the previous one in a given area. In developed countries sub-replacement fertility is any rate below approximately 2.1 children born per woman, but the threshold can be as high as 3.4 in some developing countries because of higher mortality rates.[1]