Selfish Reasons to Have More Kids is a new book by economist and blogger Bryan Caplan. It makes a simple argument of extreme importance: you should probably have more children. Though this book is written by an economist, it’s not another cute-o-nomics pop text. It’s a serious book about family planning that’s based on his reading of child development, psychology, genetics, economics, and other fields. It’s about one of life’s most important decisions, and this is what social scientsits should be thinking about.

The argument boils down to a simple point. If the evidence shows that you are over estimating the cost of having children, then, on the margin, you should probably have another child. This isn’t to say that everyone should have children, or that you should have lots of children. Rather, if you are indifferent between between having one more and not, the cautious thing to do is have one more.

Let me start with the arguments that I think are strongest. One is that people rarely regret having childen. According to survey data, people who have children rarely say that they wish that they never had children. Childless people are way more likely to say they wish they had children. Another strong argument is that having children makes the world a better place. There’s little evidence that population size by itself leads to poverty, environmental destruction, or what have you. Rather, bad policies and institutions cause these outcomes. More people means more innovators and more customers who will buy stuff from the innovators.

Another sensible argument is that you don’t need to kill yourself parenting. “The kids will be alright” should be Caplan’s motto. There’s a lot of evidence that all the crazy stuff that people do really doesn’t have much of an overall effect on life course outcomes. The piano lessons, the ballet classes – not needed. Unless the child truly enjoys these activities, and some do, better to save money, time, and stress by dropping them. Once you realize that not most kids do not need expensive inputs, you can save money and time – and have another kid.

Caplan’s biggest detractors will likely focus on his most controversial argument. He argues that you really don’t need to worry about the kids because inherited traits are much more likely to determine life course outcomes, not parenting. He supports his argument with the now voluminous literature on twins and adopted children that shows strong effects of shared parents, not family environment. Many arguments rest on his readings of these twin and adoption studies.

On one level, I agree with this overall point. We often think that we can remake people and ignore the traits, such as personality and cognitive ability, that are tough to change through socialization. As far as I can tell, twin studies do show that there are really poweful inherited traits that affect social behavior. On another level, I feel that twin and adoption studies can be pushed to far because twin and adoption studies have a very powerful, but very specific, research design.

In my view, twin studies tend to have two important limitations. First, there is non-random selection of parents into adoption. Adopters are, by definition, very unlike the rest of the population. Not in income or demographics, but in personality. Adoption is an enormous investment of resources in someone who is not biologically related to you. In other words, adopters are extraordinarily nice people. Any argument that denies the effect of parenting by appealing to studies with only Very Nice Parents is reaching too far. My hypothesis is that random assignment of twins to randomly selected parents (not just the Very Nice People) will yeild model estimates with bigger family coefficients.

The other limitation of twin and adoption studies is that they study variation in existing parenting practices. It may be the case that American parents simply don’t know how to correctly socialize a kid to reach some goal. Therefore, variations in family environment are just variations in failed practices.

Here’s a concrete example: child obesity. A hard core twin study advocate would justifiably point to twin studies showing that weight or BMI is more linked to shared parents than shared family environment. However, many Americans eat diets high in carbs, corn syrup and other ingredients. They also seem to consume many more calories than needed. To be blunt, in a world where *everyone* eats bags of twinkies, there won’t be much of an effect of living in a home where people eat a few more or less twinkies.

For that reason, it is too much of a jump to say that family environment can’t possibly affect weight. For example, parents who remove all twinkies and switch to an all broccoli diet will likely affect their children’s weight. In other words, to correctly conclude that family environment has no or little effect on weight, you would need a sample of families that have radically different diets, including at least one option that actually works (e.g., twinkies vs. broccoli). For many important life course outcomes, I am not persuaded that a sample of twins adopted into American or Western families provides enough variation in family environmnents, or that a sample would include enough families who do the practice that research has shown works.

After reading the last passage, you might think I am against genetic explanations of behavior, or that I think that Caplan’s book is fatally flawed. Instead, I see my critique as a qualification of an important argument. Even if the argument is overstated, and parents in some cases can have a big impact, parenting can be much, much less budrensome becuase the kids will be alright. In end, I find Caplan’s book to be a really humane text. Children aren’t a burden or a problem or an investment. They are to be enjoyed. They are a benefit and we should welcome more them into the world.