How a Personality Is Made: A Review of The Origins of You

If you’re fascinated by the question of why people turn out the way they do—or if you’re a regular reader of this blog—you’re probably familiar with the work of academics such as Jay Belsky and his colleagues Avshalom Caspi, Terrie E. Moffitt, and Richie Poulton. These folks have been deeply involved with several major longitudinal studies, where a set of individuals are followed from childhood to adulthood, answering detailed questions about their lives every step of the way. This type of research tells us which types of children grow up to be which types of adults, and it offers hints as to how childhood experiences can aid or hinder human development. 

In their new book The Origins of You: How Childhood Shapes Later Life, Belsky et al. outline what they have learned about human development through decades of doing this work, taking care to describe their findings in a way that is easily accessible to the general public. It’s a must-read for those intrigued by these topics, though their approach—both in their research and in how they present it—has limits.

The book covers an enormous amount of ground, but here are a few major highlights.

One study followed a group of children in Dunedin, New Zealand, for decades. It unearthed the fact that a child’s temperament as early as age three predicts many outcomes later in life, including gambling, antisocial behavior, and interpersonal relationships. It also drew connections between the way kids are parented and the way they later treat their own kids and enabled a detailed look at males who started misbehaving at different stages of their lives.

Belsky’s name, of course, is especially associated with studies of day care in America. This research also shows that kids exposed to a lot of day care are more likely to have behavioral problems, even after accounting for the usual demographic variables, and that high-quality care might increase academic achievement.

In the book’s later chapters, the authors turn to genetics. But they don’t focus on the classic methods of behavioral genetics, where researchers compare identical and fraternal twins or adoptive and biological siblings to see who’s most similar. Instead, they’re concerned with the subtle interplay of genes and the environment. For instance, their research helped to discover that the much-discussed “MAOA” gene, which has been implicated in violent tendencies, seems to have its most severe effects in individuals who were mistreated as children.

There’s a lot more here, too—about aging, tobacco, marijuana, and girls who mature quickly, to mention just a few more. And The Origins of You gives an excellent overview of this research in plain language. That alone makes it worth reading.

But there are three major limits, all of which can be addressed by reading more deeply into these topics after one finishes The Origins of You. One is that the “observational” studies Belsky et al. conduct are far from conclusive when it comes to teasing out causes and effects; a second is that, because the book is limited to the authors’ own research, it does not discuss criticisms and other approaches to these topics as much as it could; and three, the authors explain things a little too simply at times.

The typical analysis here starts with a correlation between two variables—day care and say, misbehavior. As everyone knows, a correlation doesn’t prove causation; in this case, maybe parents who put their kids in day care are more likely to be running single-parent households, for example, and maybe this family structure rather than day care itself is contributing toward the child’s misbehavior. So then the authors make a variety of statistical adjustments to rule out alternative theories for the correlation. They don’t claim to have proven causation at the end of this process, but they seem pretty satisfied.

This kind of analysis can be suggestive, but it can never really convince a skeptic.

The world is an infinitely complicated place — one where just about everything correlates in some way with everything else. A researcher can think of specific theories to disprove and specific variables to take into account, but one can never be confident that everything important is included. I tend to think genes have a powerful influence over how kids turn out, for example, and I often felt from the book that their analyses risked ascribing to the environment patterns that may be genetic in nature, a possibility they themselves concede in several contexts. (A notable exception is when the authors look at differences among identical twins.)

And even when a variable is included, it’s not fully “controlled” if it’s measured imprecisely. The data here were meticulously collected in countless interviews, but they involve lots of self-reports, subjective judgments, and, in the later chapters, weak-at-best measures of genetic propensities.

To avoid these issues, one would ideally conduct controlled experiments with random assignment—but of course, as the book notes, one cannot randomly assign a child to, say, day care rather than a stay-at-home parent. Yet the next-best thing is a “natural experiment” in which kids end up in different situations through processes that are effectivelyrandom, and in recent years social scientists have been increasingly focused on finding and exploiting such processes. The Origins of You pays little attention to this development, though, because the authors themselves have not done much of this kind of work.

Day care research is a good example of the progress that’s being made. For instance, if a government program makes subsidized day care available, you can look to see if kids’ outcomes change after the program is introduced, or if there’s a difference in outcomes between the kids who just barely made the eligibility cutoff and the kids who just barely missed it. In studies using such methods, the results have not been good for day care. These quasi-experimental approaches are far more compelling than starting with a raw correlation and trying to "control" everything that could throw it off.

My final criticism of The Origins of You is that the authors erred a little too far on the side of keeping things simple for a general audience; for example, they note that a correlation exists and doesn’t disappear when control variables are added, but they do not give precise numbers. This deprives readers of a good sense of the size of the effects and how much they decline when known confounders are taken into account. I don't think it would have required too much jargon to give at least a basic sense of these stats.

But this book does not purport to be the end-all-be-all review of the human-development literature. Instead, it's an effective and highly readable summary of the contributions these groundbreaking researchers have made to that literature. And for that reason alone, it's a great read.

Robert VerBruggen is an Institute for Family Studies research fellow and a policy writer for National Review Online.