by Bryan Caplan
FADE-OUT AND INTELLIGENCE
Twin and adoption research on young children’s intelligence always finds nurture effects. The younger the child, the more parents matter. A team of prominent behavioral geneticists looked at major adoption studies of IQ. They found moderate nurture effects for children, versus none for adults. Suppose an adoptee grows up in a family with a biological child in the 80th percentile of IQ. During his childhood, we should expect the adoptee to have a higher IQ than 58 percent of his peers. Nurture effects were largest for the youngest kids under observation, four- to six-year-olds. An average child of this age raised in a high-IQ home will typically test higher than 63 percent of his peers. Not bad—but it doesn’t last.
The Colorado Adoption Project provides an especially vivid illustration of fade-out. Nurture effects were already visible when the children were one to two years old and peaked when they were three to four years old. Toddlers adopted by parents in the 80th percentile of IQ scored about 7 percentage points higher than average. By the time the adoptees were age seven, however, two-thirds of this nurture effect was gone. By twelve, nothing was left. As the researchers bluntly concluded, “Adopted children resemble their adoptive parents slightly in early childhood but not at all in middle childhood or adolescence.”
FADE-OUT AND INCOME
Swedes have almost no financial privacy; researchers can collect your complete lifetime earnings history from tax records. One study of over 5,000 Swedish twins was therefore able to confirm that the effect of nurture on men’s income changes with age. Family has a moderate effect in your early twenties. Suppose you earn more than 80 percent of your peers. You should expect your adopted brother to make more money than 58 percent of his peers when he is twenty to twenty-two years old, and 55 percent when he is twenty-three to twenty-five years old. By the time your adopted brother reaches his late twenties, however, the effect of upbringing on income completely fades out—and remains invisible for the rest of his career. When children first become adults, their parents might find them a good job, or support them so they don’t have to work. Within a few years, however, young adults get on their own two feet, and stay there.
FADE-OUT, CRIME, AND OTHER BAD BEHAVIOR
As you may recall, a 2002 study of almost 7,000 Virginian twins found little or no effect of family environment on adult antisocial behavior. But if you read the fine print, there are noticeable nurture effects for children younger than fifteen—especially boys. This study is no fluke. A major review of twin and adoption studies of antisocial and criminal behavior finds that nurture matters most for preteens, less for children thirteen to eighteen, and least for adults. Suppose a family has a biological son and an adopted son. The biological child is a bad seed—he starts in the 80th percentile of antisocial behavior, and stays there. When his adopted brother is a child, you should expect his behavior to be worse than 57 percent of his peers. By adolescence, this shrinks to 55 percent. By adulthood, it declines further to 53 percent. The same goes for female promiscuity. Parents influence when their daughters start having sex but have little or no effect on their adult sexual behavior.
The moral: Parents are pretty good at putting their children on the right track, but not so good at keeping them there. By adulthood, it’s normal for “good kids from bad homes” to get back on track and “bad kids from good homes” to run off the rails.
FADE-OUT AND RELIGION
Twin and adoption studies of religion usually find no more than moderate nurture effects. Especially if you had a religious upbringing, this is hard to believe. To reconcile science and common sense, we simply need to distinguish short- and long-run effects. Most twin and adoption studies of religion look at adults. When researchers focus on children’s religion, the conflict with common sense goes away. The cleanest study asked over 500 Minnesota twins questions about the “centrality of religion” in their lives during their childhood and today. During childhood, the nurture effect is big. If you’re more religious than 80 percent of kids, we should expect your adopted sibling to be more religious than 68 percent. Yet by the time you’re thirtythree years old, two-thirds of this effect fades out.
When you’re raising a child, you can make him go to church, say his prayers, and shield his impressionable mind from infidels, heretics, and skeptics. When he grows up, however, he will discover the broader world. He will probably continue to pay lip service to his religious upbringing, but the effect on his behavior and beliefs will largely vanish.
Unlike Chico Marx in Duck Soup, then, twin and adoption researchers don’t have to ask, “Who you gonna believe? Me or your own eyes?” They freely admit that parents matter in the short run. If you think you’re giving your kids a head start, you’re probably correct. Your mistake is to assume that the head start lasts a lifetime. By the time your child grows up, the impact of your encouragement and nagging will largely fade away.
We often compare children to clay. When they’re soft, you can mold them into any shape you like; after they harden, they stay the way you made them. What common sense and science tell us, however, is that children are more like flexible plastic. Both respond to pressure. Yet when you remove the pressure, both tend to return to their original shape.
At some point during your childhood, you probably announced, “When I grow up, I’m going to do things my way!” Your parents might have laughed in your face, but you had the last laugh. You’re all grown up now, and I bet you do things your own way, as predicted. Your parents might object that when you were seven years old, “your way” included wearing a cape and eating only chocolate. But the obvious response is that your tastes matured because you grew up. When you were seven years old, you probably didn’t like the opposite sex much either. What changed your mind—your parents, or your hormones?
When people learn about twin and adoption research, they often object, “If this is true, how come parents haven’t figured it out for themselves?” At least part of the answer is that parents’ firsthand observation is directly misleading. When they try to mold their kids, the short-run effects are obvious to the naked eye. It is tempting to assume that these short-run effects add up—and explain why children turn into their parents. Without twin and adoption methods, we would never have discovered that the “obvious” effects of parenting are mostly a plausible, powerful illusion.
THE POWER OF PARENTING: CAN REBELLION SAVE THE DAY?
The most dangerous party member: In every party there is one who through his all too credulous avowal of the party’s principles incites the others to apostasy.
—Friedrich Nietzsche, Human, All Too Human
Twin and adoption studies usually find that the average effect of family environment is near zero. The simplest interpretation is that parents have little effect. A competing interpretation, however, is that parents have big effects, but their efforts backfire about half the time.
Rebellion is the most plausible mechanism. Almost every item on the Parental Wish List suggests compelling examples. Health: Your mom is morbidly obese; you carefully watch your weight so you don’t end up like her. Intelligence: Your dad watches sports all day long, so you seek refuge in books. Happiness: Your mom tells you to cheer up, so you sulk just to spite her. Success: Your parents are poor, so you vow with God as your witness never to go hungry again. Character: You’re habitually late to annoy your rigidly punctual dad. Values: You resolve to have at least two kids because you were a lonely only child.
Twin and adoption researchers should take rebellion more seriously. I doubt I’d be such a nerd if my dad and brother weren’t sports fans, or as slow to lose my temper if my parents had more often kept their cool. From a practical point of view, however, it doesn’t make much difference whether parenting is impotent or just backfires half the time. One story says, “Your efforts won’t work.” The other says, “Your efforts are equally likely to make things better or worse.” Either way, parents can’t reasonably expect their extra effort to pay off.
GENETIC DE
TERMINISM VERSUS PARENTAL DETERMINISM
When critics of twin and adoption methods run out of arguments, they turn to name-calling. “Genetic determinism” is their favorite epithet. Fans of twin and adoption research supposedly believe that genes completely control behavior, that heredity is destiny. Clever critics might add that my book is self-refuting: If genetic determinism is true, then our genes fully control our parenting behavior, and the best argument in the world won’t change that.
The critics of genetic determinism aren’t just wrong; they’re not even listening. Twin and adoption studies never claim that genes fully explain variation in human behavior. While they often report zero effect of shared family environment, they freely admit that they can’t make perfect predictions—even if they observe your identical twin first. Attacks on genetic determinism are a witch hunt. Witches don’t exist, and neither do genetic determinists who understand twin and adoption research.
The witch hunt against genetic determinism is particularly odd because it overlooks a truly popular dogma: parental determinism. Many imagine that the way your parents raised you is the cause of everything right or wrong in your life. When someone succeeds, they say, “His parents raised him well.” When someone fails, they huff, “Just look at the parents.” When someone fails despite his parents’ efforts, they muse, “If only his parents tried a little harder.” No matter what occurs, the root cause has to be parenting. Never mind the brute fact that kids in the same family are often quite different.
At least genetic determinism has a solid kernel of truth: Genes really do have large, lasting effects on almost every item on the Parental Wish List. Parental determinism barely has a kernel: Although parents matter quite a bit in the short run, they leave little lasting impression.
If family environment matters so little, why is human behavior so hard to predict? Because there’s a lot more to “the environment” than the family. There has to be. Otherwise, identical twins raised together would literally be identical copies. As a father of identical twins, I assure you they’re not. One of my sons is a better bicyclist, the other a better swimmer. One prefers reading, the other writing. One worries about tomorrow while the other enjoys today.
So far, researchers have failed to explain why identical twins—not to mention ordinary siblings—are so different. Discrediting popular explanations is easy, but finding credible alternatives is not. Personally, I doubt that scientists will ever account for my sons’ differences, because I think their primary source is free will. Despite genes, despite family, despite everything, human beings always have choices—and when we can make different choices, we often do. Some choices are moment-to-moment: To keep working or give up, lie or tell the truth, abandon or defend your views on immigration policy. Other choices are cumulative: You can’t change your weight, education, or income by snapping your fingers, but in the long run they depend on diet, study, and effort—all of which you’re free to choose.
Most behavioral geneticists will dismiss my armchair philosophy. Several close friends call my stance on free will “my most absurd belief.” But embracing behavioral genetics and free will at the same time is at least consistent. Remember: Another name for “unique environment” and “nonshared environment” is “none of the above.” If free will isn’t none of the above, what is?
LINGERING DOUBTS
I don’t want to come off as a “true believer” in behavioral genetics. I’m convinced, but not certain. Mediocre research occasionally gets published. When twin and adoption studies recruit one hundred subjects by mail, you shouldn’t swallow their results whole. Even excellent studies have flaws; authors often disclose them to beat critics to the punch. And flawless methods don’t guarantee clean results. The most eminent behavioral geneticists in the world occasionally shrug and say, “Well, that’s what I found. I don’t know what it means.” I’ve tried to handle these concerns the best way I know how: weeding out weaker papers, noting major shortcomings of research that made the cut, and acknowledging anomalies.
If the researchers of the next generation toned down some of their teachers’ stronger claims, it wouldn’t be shocking. We’re learning as we go. Behavioral geneticists usually assume, for example, that genetic effects are linear—two copies of a gene have twice the effect of a single copy. If this assumption is wrong, standard models oversell the effect of nature and shortchange the effect of nurture. So far, evidence of nonlinear genetic effects is mixed at best, so I ignore the issue. But better evidence may come along—and if it does, I’ll revise my position. Another doubt on the horizon: Cutting-edge studies of the children of twins find bigger effects of parenting than traditional twin and adoption studies. If the approach pans out, it won’t mean that earlier research was wrong, but nurture will merit a bit more credit.
The most important weakness of behavioral genetics, though, is simply that research focuses on middle-class families in First World countries. The results might not generalize. Twin and adoption studies almost never look at people in Third World countries. So you shouldn’t conclude that Haitian orphans would turn out the same way if raised in Sweden. Twin and adoption studies also tend to ignore the poor in First World countries. Twins come from all walks of life, but it’s usually harder to get twins from lower-class homes to join a study. Adoptees almost never grow up in lower-class homes, even though their biological parents tend to be lower class. Don’t assume that poor inner-city kids would turn out the same way if they grew up in a sheltered suburb.
For policymakers, the restricted range in twin and adoption studies is a major blind spot. But middle-class parents in First World countries needn’t worry. Families like yours have been studied to death. In your corner of the world, you can safely rely on the postcard version of behavioral genetics: The chief cause of family resemblance is heredity, not upbringing—and while the short-run effects of upbringing are self-evident, they leave little lasting impression.
WHAT THE SCIENCE OF NATURE AND NURTURE MEANS FOR PARENTS
Parents want the best for their children, but parenting has surprisingly little effect on whether or not children get the best. While this discovery might seem too dangerous for human consumption, it’s great news for parents and children alike. False hope in the power of nurture leads to wasted effort and wasted opportunities. Parents who understand the facts can help their families and themselves.
LIGHTEN UP
Let’s start with the obvious. If parental investments don’t pay off, then relaxed parenting is a free lunch: better for parents, and no worse for kids. If you falsely believe that your child’s health, intelligence, happiness, success, character, and values heavily hinge on your effort, you will waste sweat and tears. Once you accept the truth, you can drop a lot of painful parenting.
How much pain can parents safely avoid? To answer this question, remember the main limitation of twin and adoption studies: They almost always focus on relatively normal families in relatively rich countries. You can’t read an American adoption study and conclude that abject poverty and severe neglect do no lasting damage. But you can conclude that it’s okay for parents to move within the normal American range. You don’t need to fret about whether to sacrifice more for your children than 20, 50, or 80 percent of parents like you. If the typical adoption agency would consider you a suitable parent, you’re good enough to let your children to reach their potential. A good rule of thumb: If your parenting style passes the laugh test, your kids will be fine.
When they hear about the evidence from twin and adoption research, parents occasionally respond, “I know I don’t matter much, but every little bit helps.” But how much pain are they willing to bear for a small gain? For each of the items on the Parental Wish List, there’s a big difference between homes with kids in the 50th percentile and homes with kids in the 80th. Suppose you want to turn your child into a churchgoer, even though you’re not especially religious. In 2006, the General Social Survey found that the median American went to church “several time
s a year.” But one person in five went every week—or more. Moving from the 50th to the 80th percentile on this one trait therefore takes almost fifty extra services a year. If you don’t really enjoy church, that’s a lot of effort to make your kids 2 or 3 percentage points more religious.
Or take education. The Korean adoption study found that if a mom has an extra year of education, her adoptee gets five more weeks. To raise her child’s educational attainment by a single year, a mom would need more than a decade of extra schooling. That’s practically an entire childhood. Even ignoring the massive cost for the mother, it’s a lonely life for the child.
I see why parents of grown children resist my message. They’re proud of their kids and don’t want some genetic accountant telling them to write off their investments. What’s puzzling is the resistance of people who are still midstream. The exhausted parents of a two-year-old should be thrilled to hear that the next sixteen years of parenting can be less of a chore.
