Why people have only one child?

Families may have an only child for many reasons, that includes:

Personal preference, family planning, financial and emotional or physical health issues, desire to travel, family stress, educational advantages, late marriage, stability, focus, time constraints, fear over pregnancy, advanced age, an illegitimate birth, infertility, divorce, and death of a sibling or parent.

The premature death of one parent also contributed to a small percentage of marriages having only one child until the mid-twentieth century, not to mention the then-rare occurrence of Gay marriages.

Families in general are extremely diverse, and there are few reasons why parents would choose to have only one child.

Because the question presumes that there is a choice, we obviously cannot consider circumstances such as the inability to conceive another child. 

However, there are numerous reasons why parents choose to have only one child.

Here are a few of the most important reasons that come to mind for me:

1. Children are expensive. let’s face it!Each additional child is going to run the parents about $250,000. Some parents might not think that far ahead, but most good parents are already thinking about college when their kids are entering kindergarten.
2. Health/medical conditions. As mentioned in many other answers, this is a big deal. My son has ADHD and her younger years were a tremendous struggle. Not only do children with health issues require a lot more work, but when there is a significant possibility that subsequent children will inherit a similar condition, it really makes you think hard about having another kid.
3. Because having one kid is great! Some people love to whine about only children as if they were somehow going to topple society as we know it. Many of these people believe every pseudo-scientific “study” they read on Facebook that suits their argument. There are unique challenges to raising an only child, but there are also unique challenges to raising three or four children. As long as you are taking care of your kids, you should be able to have as many of them as you want, be it one or ten. In some ways, I know my son would benefit from having a sibling, but in other ways it would present a lot of new challenges.
4. Population. The world population is growing like crazy. Not everyone feels the need to contribute several more little creatures to this planet.
5. Age of parents. Parents are getting older and older. But later in life it can be complicated and dangerous to conceive. Some parents might want to wait until later in life to have a kid, but at some point it becomes too late to keep making babies, or they just don’t have the energy or desire to do so.
6. Relationship issues. I haven’t seen this one touched on much in other answers, but I would posit that it’s a big factor. With the prevalence of divorce and other complex family issues, two parents who were deeply in love and purposefully had their first kid with the intent of spending their lives together may find their relationship on the rocks a few years later; obviously at that point having another kid isn’t a wise choice.
7. The first Child one wasn’t planned to begin with. Okay, another reason that hasn’t been touched on much. Let’s face it, most kids aren’t planned. They just happen. If you didn’t plan on having one kid, why would you plan on the second? Even if you love kids, sometimes it takes the nudge of fate (and maybe some bad choices or a few too many drinks) to turn it into a reality.