As a parent educator, I find the topic of [over]population a bit touchy. Though some people choose to build their families through adoption, most (including me) do so by procreating. The topic of how many people is enough, or too many, is not likely to find a very objective ear among the people I most often talk to. I also think that the urge to have biological children is strong and compelling, and difficult to deny.
On the other hand, I have often found it curious that the largest families I know also often make the smallest collective impact on our planet. They raise much if not most of their own food, they tend to consume fewer resources out of choice and necessity, they travel less...they just seem to live more simply and sustainably.
It seems obvious that the more people who live on this planet, the more pressure we will put on the system in many, many ways. However, it is not just a question of how many people, but also how they live, that impacts this equation. The resources required to support one average American person could easily support over three dozen people in some of the poorer countries in Africa. As the 2 billion plus people in India and China begin to have access to luxuries that we in the US have come to consider necessities...well, I don't know what will happen, but it's not going to be part of the solution when with respect to the problem of global scarcity. Population and consumption are intimately and inextricably linked.
Most of the books I've read on the topic of population take a gloom-and-doom position - that the current trends in population that could top out at nine or ten billion (or, according to some sources, more) in the next few decades, will lead to global famine and war. That may be true, but since people are not likely to stop having sex, and are not likely to stop having babies, what can we do?
More: Population, Nature, and What Women Want by Robert Engelman is a refreshingly hopeful examination of the issue of population that is written for the general reader. According to Engelman, if given choices about when and how often to bear children, women will choose to have smaller families, particularly during times of scarcity. He argues that human numbers were held fairly constant for most of our history through a combination of folk methods of birth control and (unfortunately) high death rates. However, once choices about family size became political and/or religious, combined with the ability of science to help people live longer, our numbers surged and we haven't looked back.
In order to stem the tide of population growth, Engelman proposes that we simply need to put choices about childbearing into the hands of those most equipped to make them - families, specifically women - through reproductive health counseling, widespread and affordable access to reliable birth control, and availability of abortion when necessary. Will this stop the runaway growth of the human species in time to avoid disaster? Maybe; maybe not. But, current strategies are clearly not doing the trick, and as as the author states in his cleverly-titled chapter "Zen and the Art of Population Maintenance", "the safe and healthy reproductive habits [reproductive health workers] promote do create conditions out of which population stability tends to arise (179)."
As an interesting aside, Engelman suggests that the current prohibition against co-sleeping with infants came from a time when birth control and abortion were unavailable and infantacide was the only real option for women looking to control their family sizes. These women began to "accidentally" overlay their babies during sleep, leading the Church to forbid co-sleeping, an attitude that continues today in many industrialized cultures. Hmmmmm....