Happy, Healthy 101: A Parent’s Guide to Helping Your Child Thrive Across the College Years

Chapter 4: You get what you get, and you don’t throw a fit: How your child’s genes influence their behavior

There is a saying in my field (which is called behavioral genetics) that everyone is an environmentalist until they have their second child. Having a second child is a humbling experience because you realize that the same child-rearing practices can result in very different outcomes in different children. That strict sleep schedule that you implemented with baby number 1 that led to them sleeping through the night at week 2, and for which you congratulated yourself over your amazing parenting skills — baby number 2 could care less about the sleep routine and doesn’t sleep through the night for the first year. The timeouts that led your first child to quickly stop their mischievous behavior and fall in line; child number 2 will engage you in an hour of cat-can-mouse, refusing to sit their bottom on the timeout chair. Or perhaps you had the opposite experience, where your first child seemed not to respond to the various environmental manipulations that seemed to work for all your friends and you were questioning your ability as a parent. Cue baby number 2 and suddenly it works like a charm. The bottom line is that raising more than one child underscores how different children are. They clearly are not simply products of their environmental upbringing. They come with their own temperaments and personalities. They are not blank slates waiting for you to mold them into the lovely human beings you want them to become.

When I had my son, I too was originally seduced by the “power of my parenting”. Not only did I have training across developmental and clinical psychology, but being the consummate academic, I also read a ton of parenting books before my child was born. My child was an amazing sleeper and a textbook easy baby. I thought I had this whole parenting gig in the bag. I remember my son throwing a temper tantrum at 15 months and looking at him and thinking in a distant, academic way, “I am actually not enjoying my child right now.” It was the first time in 15 months I had ever experienced anything other than pure joy associated with my child. Wow, was I in for a surprise. It turns out that his challenging temperament was just starting to manifest (research suggests that consistent temperamental characteristics start to emerge around age 2–3), and it was a long 10 years after that. I even wrote a book about it (The Child Code: Understanding your child’s unique nature for happier, more effective parenting, Penguin Random House, 2021), to help other parents understand just how much genes influence kids’ behavior, and how we can use that information to simplify our parenting. I’m happy to say that at 16, with a little more brain development under his belt, he’s shaping into a lovely young man. That said, no matter how many parenting books I read (or write!), I won’t pretend that I’ve successfully molded him into a perfectly behaved child.

So here’s the point: part of parenting is recognizing and embracing your child for their own unique characteristics and temperament, and recognizing the limits of your parenting. That may be strange to read in a blog for parents, but remember my area of study is genetic and environmental influences on adolescent and young adult outcomes. We cannot deny the strong role that biology plays in our children’s dispositions and choices. Staying involved in your child’s life and helping them navigate the big decisions of emerging adulthood does not guarantee that they will turn out exactly as you want. Children have their own temperaments and ultimately control their own behavior. That said, acknowledging the role of genetic predispositions on behavioral outcomes does not negate the role of the environment, or parents. If you’re interested in learning all about how this works, all the details are in my book The Child Code. Here I present a quick overview of the field of behavior genetics, which is a field devoted to understanding to what extent different behaviors (think: how much your child drinks, how impulsive or anxious they are, etc.) are influenced by genetic and environmental factors. Yes, this is the field that studies the so-called nature/nurture debate, although as any informed scientist will tell you: there is no debate, it is clearly both, always. Debate over. Behavior geneticists are now freed to study much more interesting questions about how genes and environments influence behavior across development.

What we know from the last few decades of research is that virtually all behavior is genetically influenced to one degree or another. There are a series of study designs that allowed scientists to figure this out (note: if you don’t care how scientists figure this stuff out and just want to take my word for it, skip ahead to the next chapter!).

The first step in figuring out if a trait is genetically influenced is conducting family studies. If something is influenced by genes it should run in families, since children are a combination of their biological parents, sharing 50% of their genetic variation with their mother and 50% with their father. But the problem with family studies is that we usually spend the most time with people who are more closely genetically related to us. For example, siblings (who share on average 50% of their genetic variation) usually spend more time together than cousins (who share on average 12.5% of their genetic variation). So when something “runs in a family” it could be genetically influenced, but it might also be environmentally influenced.

In order to tease apart the effect of genes and the environment we need other study designs. This is where adoption studies and twin studies have been critical. There are many variations on the adoption design, but in it’s simplest form, researchers compare how similar an adopted individual is to their biological parents (with whom they share genes, but, if adopted away at birth, no environment) as compared to their adoptive parents (with whom they share an environment, but, assuming they are not a relative, no genes). One of the most surprising findings to emerge from adoption studies was how much adopted children resembled their biological parents, with whom they had no contact, more so than their adoptive parents who were raising them. These studies provided clear and compelling evidence that genes were important for behavioral outcomes like substance use, aggression, and mental health challenges.

The problem with adoption studies is that they are increasingly difficult to carry out. Many adoptions are now open, meaning that individuals have contact with their biological parents, blurring the clean separation of parents with whom one shares genes but no environment, and parents with whom one shares an environment but no genes. In addition, adoptive parents and biological parents who give their children up for adoption tend not to be representative of the population. On average, adoptive parents tend to be better educated and well-off, and have lower rates of mental health challenges, in part due to the extensive screening process that adoption agencies carry out. Parents who give their children up for adoption are on average, skewed the other direction, being more likely to have less education, lower incomes, and more likely to have substance use or mental health challenges. In addition, many adoption agencies are reluctant to release information about biological parents to researchers.

This brings us to the most widely used study design in behavior genetics: the twin design. Twins basically come in two “types”: identical twins, scientifically termed monozygotic twins (MZs) who share all of their genetic variation (they are essentially “clones” of one another) and fraternal twins, called dizygotic twins (DZs) who share just half of their genetic variation like ordinary siblings. MZ twins result when a single egg is fertilized by a single sperm, but at some point during cell division the zygote splits into two. It is unclear what contributes to MZ twinning, although in vitro fertilization can result in increased MZ twinning, perhaps due to instability of the zygote. DZ twins result from two eggs fertilized by two sperm. They result when multiple eggs are ovulated by the mother. This is the kind of twinning that “runs in families”. This is because hormone levels influence the likelihood that multiple eggs will be ovulated, and hormone levels are known to be genetically influenced. The other major (natural) factor related to DZ twinning is advanced maternal age. Older mothers are more likely to ovulate multiple eggs, and therefore have higher rates of DZ twinning. Of course the chance of DZ twins are also increased by in vitro fertilization when multiple fertilized eggs are implanted.

The fascinating thing about twins is that they allow a natural study design to evaluate the extent to which a behavior or outcome is genetically and environmentally influenced. If something is entirely influenced by the home environment, DZ twins should be just as similar as MZ twins. For example, if being raised by a parent with an alcohol problem leads to children developing alcohol problems themselves, then you would expect DZ twins to be just as similar to one another as MZ twins, regardless of the fact that they differ in how genetically related they are. The key factor in this example is the environment. In contrast, if alcohol problems are genetically influenced, you would expect MZ twins to be more similar to one another than DZ twins, because though both types of twins share their home environments, MZ twins are more similar genetically. Finally, because MZ twins share all of their genetic variation, and their home environment, if they are not exactly identical, we know that other unshared environmental factors must be important. This could be anything from a traumatic event or injury that one twin experienced and the other didn’t, to a different group of friends that one twin has.

At this point there have been twin studies done on probably about any behavior you can think of, including biomedical conditions, psychiatric and substance use disorders, personality, and even things like social attitudes, life satisfaction, divorce, and voting behavior. The overwhelming conclusion from thousands of studies is that virtually all behavior is genetically influenced to one degree or another. A good rule of thumb is that most substance use and mental health behaviors are about 50% heritable, meaning that about half of the differences between people — in how much they drink, smoke, worry, are depressed, are extraverted versus introverted, etc. — is due to differences in their genes. The other half of what contributes to differences between people is differences in environments, the influence of parents, but also of friends, schools, community, society, and even the slings and arrows of outrageous fortune, the random things that happen to us and impact our lives.

What does all this mean for you as a parent? It means that your child’s life outcomes are not solely in your hands. That can be both liberating, and frightening. It means that it’s not all on your shoulders — for better and worse. Genes influence brain development (as does the environment) and that’s why our kids all start life wired differently. It’s why differences in temperament and personality emerge early in childhood. It’s why our kids respond differently to our parenting. And it’s why forcing our children into the mold that we might have imagined for them very often doesn’t work. They come with their own unique codes, that shape their natural dispositions and their life choices. As they set off on their own, their ability to select their environments and affect their outcomes increases. Research shows that the importance of genetic dispositions steadily increases as kids gain autonomy. But there’s still room for us parents to help them make the best choices and grow into the best version of themselves. And that’s what the last chapter is all about.

Sidebar: if you’re interested in learning more about the many ways that our kids’ genes influence their behavior, I encourage you to check out my book The Child Code. It has surveys for you to complete to identify your child’s temperament, along with parenting strategies that work better for kids with different dispositions. Even if you have a child who is now past “childhood” and entering young adulthood, it can help you better understand your child and build a stronger relationship with them as they grow into an adult.

Up next…

Chapter 5 How to Talk to Teenagers in a Way that Actually Motivates Behavior Change

Summarize

Happy, Healthy 101: A Parent’s Guide to Helping Your Child Thrive Across the College Years

Chapter 4: You get what you get, and you don’t throw a fit: How your child’s genes influence their behavior