We have four children, I just had my tubes tied after dd was born (we also have 3ds). I thought long and hard about how many we would have, dh didn't seem to have a preference
and decided I would just "know" when the time was right to stop having children. We were done with three, but dd was determined to come through after all.
I think the point was already made about how to live a conciencious lifestyle, but I also believe we choose who we are meant to spend our lives with in this realm. Forgive me if I get too spiritual. Each one of us has lessons to learn and challenges to overcome. Part of my decision to have more than one or two children stemmed from my own life growing up with a sister who was 10 years older than me, and we didn't get along. I always ached for a sibling who was closer to me, or at least another I could turn to in good times or bad. I wanted my children to remember happy together family times, not times playing alone. That said, I am not against only having one child, this was just my experience.
If we concentrate more on making life rich for every child, I think this issue about how many we should have would take a back seat. We have far fewer resources than many of our friends who only have one or two, but I like it that way. Many, I'm sure, would be of the opinion that we are too poor for our family size (you name the gov't program, we qualify for it) but we also don't think we were meant to provide things like college funds or expensive clothes for our children. I appreciate the BA I have more so b/c of the hard work it took for me to earn it. Dh and I will enjoy the fruits of our own labor into our old age (hopefully), but the children will have to labor for themselves, including working by the age of 16 and paying for the things they want themselves. I paid rent any month I was home after I turned 18. It made me very responsible. I no longer rec'd an allowance or spending $$ after the DAY I turned 16, and had to cover the cost of lunches bought at school, my bus ticket, etc... as well as any clothing outside of my school uniform. Our children will either go to a private highschool or be homeschooled, and either way will hopefully leave a positive impression wherever their lives take them.
As far as the low infant mortality rate in the US, this isn't entirely true. We actually have one of the highest in the industrialized world. Mostly due to our severe problem with interventions at birth.