In 1990, Thomas J. Bouchard, Jr. and his colleagues at the University of Minnesota published a striking finding: About 70 percent of the variance in IQ found in their particular sample of identical twins was found to be associated with genetic variation. Furthermore, identical twins reared apart were eerily similar to identical twins reared together on various measures of personality, occupational and leisure-time interests, and social attitudes.
Bouchard's study, along with many others, has painted a consistent picture: Genes matter. The studies say nothing about how they matter, or which genes matter, but they show quite convincingly that they indeed do matter. Genes vary within any group of people (even among the inhabitants of middle-class in Western society), and this variation contributes to variations in these people's behaviors.
Let's be clear: Twin studies have received much criticism. Even though the proliferation of advanced statistical techniques (such as structural equation modeling) and the implementation of additional controls have allayed some of the concerns, they haven't allayed all of the them.
Even so, the findings from twin studies should not be understated; it counters many a prevailing belief that we are born into this world as blank slates, completely at the mercy of the external environment. Because our psychological characteristics reflect the physical structures of our brains and because our genes contribute to those physical structures, there are unlikely to be any psychological characteristics that are completely unaffected by our DNA.
The fact that both our genes and our environment contribute to who we are and depend on each other is actually quite a good thing! Give too much control to our environment or our genes, and we lose free will. The way we work gives us choice.
Unfortunately, findings from twin studies are often misunderstood, misinterpreted and blown out of proportion -- not just by the media, but even by serious scientists who get their work published. To put things in perspective, I teamed up with the well-known developmental psychologist at Pitzer College, David S. Moore, to list eight facts about genes, twin studies, and the heritability statistic that may come as a surprise to many people -- even biologists!
Twin studies partition the variance in nature and the variance in nurture. This allows researchers to determine whether differences in genes or differences in the environment in a particular population are associated with more of the differences in observed behavior.
In reality, all biological and psychological characteristics are constructed during development, when genes interact with local environmental factors that can be influenced by the broader environment. Therefore, gene-environment interactions are understood to drive the development of all of our characteristics. Naked DNA (or RNA) is simply not sufficient to produce psychological or biological traits.
Therefore, when it comes to understanding the development of a trait in a particular person, nature can never be separated from nurture. Science writer Matt Ridley has put it this way:
[Genes] are devices for extracting information from the environment. Every minute, every second, the pattern of genes being expressed in your brain changes, often in direct or indirect response to events outside the body. Genes are the mechanisms of experience.
The authors of many twin studies have claimed that the environments experienced by twins (or any two siblings) do little to create differences in intelligence and personality as adults. Somehow, this finding translates to the media as "parenting doesn't matter." This couldn't be farther from the truth.
Take the most essential element: a child needs to be raised in a family, almost any kind of family, to develop the ability to speak a language. Since every single person in twin studies checks that box -- that is, they are raised in a family of some sort -- this factor never varies and thus does not predict differences in ability to speak a language. But does this mean that the variable "has a family" doesn't matter in determining whether or not a person develops the ability to speak a language? Of course not! That's like saying that water has no influence on a fish's development because all fish live in water. Just because a variable doesn't vary doesn't mean it has no causal impact on a particular outcome.
The parenting factors that are statistically associated with differences between individuals should never be confused with the parenting factors that cause the development of a trait within an individual. Genes could "account for" 100 percent of the variability in a trait in a particular twin study, but this does not mean that environmental factors are therefore unimportant in the development of the trait; parents still matter and will always matter.
It turns out that parenting matters, just in a way different than originally assumed. Genes matter to the extent that they support parenting, because like any other behavior, parenting behaviors are influenced by the genes. Parents matter to the extent that they support the expression of genes.
None of the twins in Bouchard's study were reared in real poverty, were raised by illiterate parents, or were mentally retarded. There is reason to believe that under more dire circumstances, the heritability of IQ would be significantly lower than that reported by Bouchard. After all, if everyone were raised in an identical environment, variations in their psychological characteristics couldn't possibly be accounted for by anything other than variations in their genes (since there would be no variations in their developmental environments); the more variation in environments that twins in twin studies are exposed to, the lower the heritabilities we should expect to find.
In one study, Eric Turkheimer and colleagues studied 320 pairs of 7-year-old twins who were raised in extreme poverty. Among the poorest, the shared environment accounted for most of the differences in IQ (60 percent), and the genes accounted for very little. Consequently, in this study, the heritability of IQ was reported to be close to zero! Among the richest, however, the heritability of IQ approached what Bouchard found: Variations in the genes accounted for most of the differences in IQ scores, and the shared environment accounted for very little of the variance. This study points to the fact that estimates of heritability depend on the sample that is studied, and the environment of that sample.
Turkheimer's study should also be a reminder that just because something is heritable doesn't mean it's immutable. The Flynn effect -- the dramatic rise in IQ witnessed in the 20th century -- is a good example of that. The Flynn effect should be a reminder of just how much the environment matters, even after completely controlling for genes (by looking at IQ changes across generations).
This raises a deeper point: Depending on what you hold constant, you can either show a genetic contribution or an environmental contribution. The point is that both are always contributing to the development of any trait, and context matters in which accounts for more of the differences in a trait.
The heritability of a trait can vary from 0.00 to 1.00, depending on the environments from which research participants are sampled. Because we know that genes play some role in the development of any trait, the precise heritability estimate doesn't matter in a practical sense.
Heritability depends on the amount of variability in the environmental factors that contribute to a trait. The problem is that our understanding of the factors that contribute to the development of human traits in general -- and to IQ in particular -- is currently so deficient that we typically do not know if the environmental factors important in the development of a particular trait are stable across testing situations, vary somewhat across those situations, or vary wildly across those situations.
Even if a population of individuals were to develop in a range of environments believed to be the same as that in which a particular study was conducted, the results of that study would not allow us to predict developmental outcomes in the new range of environments because the environmental factors that the researchers originally focused on -- and controlled for -- might not be the relevant environmental factors at all.
Instead, the crucial environmental factors might remain unmeasured, and consequently, variability of those factors across the new range of environments could easily be very different than the variability of those factors across the environments sampled in the original study.
Of course, we could just aim to measure all of the environmental factors that might affect the development of a trait. But it is not at all obvious prior to developmental analysis which environmental factors might make important contributions to the development of specific traits, so that approach would leave us measuring a seriously unwieldy number of variables.
Because the development of behavioral and psychological characteristics can be influenced by experiential factors in ways that are unpredictable from casual observation, we cannot hope to happen to measure -- through sheer lucky guesswork -- which environmental factors contribute importantly to the development of those characteristics; we first need to understand the mechanisms by which those traits develop.
Environmental factors influence the development of highly heritable traits just as much as they influence the development of non-heritable traits (i.e. a trait like height, which is highly heritable in most developed nations, is very affected by environmental factors, like diet). Likewise, as can be seen from the example below, genetic factors influence the development of non-heritable traits just as much as they influence the development of highly heritable traits.
In fact, the least heritable features of human nature may be those that appear to be the most genetically determined! Consider the fact that having 5 fingers on each of our hands is not a particularly heritable characteristic (because most finger number variations in humans are attributable not to genetic variation, but to variations in experiences, such as accidents). Nonetheless, it is quite obvious that genetic factors play a role in determining the number of fingers we have on each of our hands!
Because heritability is a population statistic, it has nothing to say about the individual. It makes no sense to ask whether a particular individual's intelligence has been more determined by nature or by nurture. As already stated, every trait develops through the interplay of genes and the environment. Nature and nurture are complementary, not at odds.
Because adoption and twin studies that seek to account for trait variation in terms of genetic and environmental variation are always correlational, they reveal nothing about the causes of the appearance of the traits.
Adoption studies and twin studies do not entail the purposeful manipulation of either specific genes or specific environmental factors. Hence, such studies are unable to generate satisfying understandings of the factors and processes that contribute to the development of intelligence.
It's important to keep in mind that the route from genotype (genetic makeup) to phenotype (observed behavior) is hardly ever clear-cut. It's possible for many traits to involve gene-environment correlations. The idea here is that environments set off an appetite in the genes that nudges individuals to engage in certain experiences, and the environment then responds in a reciprocal fashion that reinforces an individual's nature. The genes and environment eventually become correlated.
It's very easy to imagine how slight genetic predispositions can get magnified through the course of development by the environment. Imagine if you were born slightly taller than others (maybe you don't have to imagine this!). You get picked first for the basketball team, whereas your smaller friends may not get picked at all. This would give you more experience in basketball, which increase the chances you'd get picked first for another team. Those who repeatedly don't get picked for the basketball team may invest in other skills, such as physics or art. This cycle continues to magnify observed ability differences in basketball between those who keep getting opportunities to increase their skills and those who didn't get picked that first time. The rich tend to get rich, and the poorer tend to get poorer. The causal route from genes to behavior is often very complex!
Heritability does not tell us how likely it is that people's characteristics will be inherited by their children. Because traits that are 100 percent heritable can nonetheless be strongly influenced by environmental factors, it is not the case that a trait found to be heritable in a particular twin study will be passed from a given pair of parents to their children. Let's imagine that a study of alcoholism in the United States finds that the vast majority of the variation in people's tendencies to drink to excess can be accounted for by variation in their genes. If we then take a baby, newly born to a pair of alcoholic American parents, and raise it in a small village in southern India where it never encounters alcohol across its lifespan, it will not develop alcoholism. We often talk as if we "inherit" full-blown traits from our parents, like eye colors, nose shapes and shyness. But all that we actually inherit from our parents are our genes and our genes' (and our) environments, factors that then construct full-blown traits during development. Consequently, it doesn't matter how heritable a trait is; if development of the offspring occurs in a different environment than the parent developed in, most bets are off.
Does Heritability Have Any Practicality?
We hope these eight facts have cleared up some misunderstandings. After reading these facts, it might be reasonable to ask, "Does the heritability coefficient have any practical value?"
At the very least, heritability tells us how much of the variation in IQ can be accounted for by variation in genetic factors when development occurs in an exquisitely specific range of environments. However, David S. Moore has argued that even this is not significant when we realize that the magnitude of any heritability statistic reflects the extent of variation in unidentified non-genetic factors that contribute to the development of the trait in question.
Because we cannot assess the variability (across our testing environments) of all the yet-to-be-identified non-genetic factors that influence IQ, Moore argues that estimates of the heritability of IQ are effectively uninterpretable and unable to be applied in any appropriate way. As Moore puts it in his journal article:
Many psychologists continue to compute heritability statistics without questioning what exactly it is that they reveal to us. Unfortunately, careful consideration of these statistics suggests that they might not be applicable in any meaningful way, and so, consequently, are uninteresting at best and misleading at worst.
Some of the most well-known behavioral geneticists, including Thomas Bouchard, Jr., recognize that it's time to move beyond heritability estimates. A currently active area of research is the study of epigenetics, and how the many interacting genes that make up any trait are differentially activated depending on the environment. Nature and nurture are inextricably intertwined, and it's time for science to figure out how.
Recommended Reading
Bouchard, , T.J., Lykken, D.T., McGue, M., Segal, N.L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of twins reared apart. Science, 250, 223-228.
Deary, I.J., Penke, L., & Johnson, W. (2010). The neuroscience of human intelligence differences. Nature Reviews: Neuroscience, 11, 201-211.
Harris, J.R. (1999). The Nurture Assumption: Why Children Turn Out The Way They Do. New York, NY: Free Press.
Johnson, W. (2007). Genetic and environmental influences on behavior: Capturing all the interplay. Psychological Review, 114, 423-440.
Johnson, W., Turkheimer, E., Gottesman, I.I., & Bouchard, T.J., Jr. (2009). Beyond heritability: Twin studies in behavioral research. Current Directions in Psychological Science, 4, 217-220.
Moore, D.S.. (2003) The Dependent Gene: The Fallacy of "Nature vs. Nurture." New York, NY: Times Books/Henry Holt & Co.
Moore, D.S. (2006). A very little bit of knowledge: Re-evaluating the meaning of the heritability of IQ. Human Development, 49, 347-353.
Pinker, S. (2003). The Blank Slate: The Modern Denial of Human Nature. New York, NY: Viking.
Ridley, M. (2004). Nature via Nurture: Genes, Experience, and What Makes Us Human. New York, NY: HarperCollins.
Turkheimer, E., Haley, A., Waldron, M., Onofio, B, & Gottesman, I.I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14, 623-628.
Follow Scott Barry Kaufman, Ph.D. on Twitter: www.twitter.com/sbkaufman
Dr. Frank Lipman: Is Our Health Determined By Our Genes?
Nature versus nurture - Wikipedia, the free encyclopedia
Nature vs Nurture - How Heredity and Environment Shape Who We Are
Nature vs. Nurture: Mysteries of Individuality Unraveled | LiveScience
What's surprising is the number of errors in this post. Take, for example, point 6. You say:
“Because heritability is a population statistic, it has nothing to say about the individual. It makes no sense to ask whether a particular individual's intelligence has been more determined by nature or by nurture.“
As Tal 2009 (“From heritability to probability“). has shown, heritability estimates allow one to make a probabilistic statement about the factors underlying an individual’s deviation from the population mean. For example, we can say that, for a particular individual with an IQ of 120 who’s population has a heritability of IQ of 0.8, there is an 84% chance that the individual’s deviation from the mean is due more to genetic than environmental factor.
Could my parenting affect them? Yes. And yes to everything they have encountered in the world. But they had their own unique set of qualities from the get-go!
Second, identical twins adopted by different parents have three things in common:
1. DNA
2. The environment in the womb, where MOST of their brain development occured.
3. They were both adopted by parents who qualified under adoption criteria in the same jurisdiction, and who both wanted to adopt an infant.
So how can anyone say a correlation is based on DNA alone?
the shared fetal environment generally makes twins more different from each other, not more similar. Moreover, in twin studies monozygotic twins are usually compared to dizygotic twins, and the usual finding is that the former are much more similar to each other than the latter, even though both types of twins share the same womb.
Another piece of evidence for the strong heritability of IQ (and other traits) is that adopted children resemble their biological relatives more than their adoptive family. In fact, the adult IQs of two biologically unrelated children raised in the same home are not more similar than those of two random persons from the same population.
Moreover, it is now possible to measure the actual extent to which siblings share genes (it's 50 percent on average, but varies somewhat), and then study if genetically more similar siblings have more similar IQs. This method does not have most of the alleged shortcomings of the twin method. See this paper where they used this new method to corroborate the twin study finding that the heritability of height is about 80 percent: http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.0020041
But the genetics of human eye color are very well understood, conforming to the Mendelian model. Brown is triggered by a dominant gene, blue and green are each caused by recessive genes, and you need two recessives (one copy from each parent) to manifest blue or green. If heritability is *so* tied to environment in all cases, what environmental factors would cause a child's eyes to develop a different color from the one we "expect" based on inherited genes? I can only think of mutagens in the environment (chemical, radiation, etc.) to explain something like this. In general, though, this outcome isn't expected.
Your comments about eye color are actually not correct. We can *think* about eye color being determined this way if we want to, because the Mendelian story works reasonably well for prediction purposes. But if you start looking at the molecular biology, it's not in fact the case that eye color is determined by single genes received from our parents. This is a simplification that can be useful (in particular, to teach kids about genetics in school) but the truth is that it's a lot more complicated than the traditional Mendelian story makes it out to be. If you'd like me to steer you to some readings on the topic, just let me know.
Hey Dr. Kaufman, if both your parents are both caucasian you will be too, if they are cats you will be a cat, no matter the environment, poverty, nurturance or locale, My huge blonde friend has huger and blonder brothers and parents than I do and I think that is the probably reason why he is huger --not something different that I ate. What are you saying?
Almost all twins studies (separately adopted at birth) reflect their IQs are closer to each other and their biological parents than to their adoptive parents. The list of personality characteristic and other non-physical attributes is long and surprising too. It is true however that the environment can actually change which genes are expressed (epigenetics), which change can be passed to the next generation in their changed form.
We probably have a "range" that is genetically determined. Free will works best when one has optimal genetics to go along with it. An optimal brain does have more free will than one that is not compromised by illness, disease, viruses, bacteria or genetics. Every brain has limitations--no matter what the parents do or what the non biological environment is.
Until we measure everyone's brains and examine/define all genes and variations, no study that discredits genetic influence can be considered accurate. Genes matter. A lot. Environment only counts when the genetics are the least compromised.
No, it doesn't mean that at all. The problems with twin studies are numerous. Here are just two of them:
First, even when raised apart, twins usually grow up in very similar homes, because they tend to get adopted by the same kinds of people, from similar backgrounds, socioeconomic status, and education themselves. So there is little true variation to their upbringing.
Second, and much more important, during the most influential period of development, their 9 months of gestation, they still share the exact same environment. Studies on the genetics of behavior in other mammals, using in vitro fertilization and gestation of siblings in different surrogate mothers, show that even behaviors long believed to be entirely genetic, because of previous experiments similar to the twin studies discussed in the article, are in fact environmental. That early environment is tremendously influential on later behavioral development. To ignore the single most influential portion of a person's development, and the influence of environment during that time, is just stupid.
For this reason, twin studies are virtually worthless in this regard and only continue to get used because we can't ethically carry out the same kind of in vitro experiments in humans. But all evidence from other mammals, where we have done these experiments, shows virtually no support whatsoever for any significant genetic basis for complex behaviors or intellectual ability.
But we can, and have, done them on other mammals. And when we do, those twin effects that are often supposed as genetic in humans seldom if ever hold up. Twin "adoption" studies in other mammals, where genetically identical siblings are raised by different parents have long identified many such supposed genetically controlled behavioral traits, with even better resolution than any of the human studies have (because of better control). But when in vitro fertilization has removed the common fetal environment, those genetically identical siblings show the same variation in behavioral traits that non-related individuals do.
Yes, if you look at the human studies, they are all very similar. But that's the problem - they are all also similarly flawed, so one would expect that they all give similarly flawed results.
From a different angle: Genetic determinism is one of the most dangerous and deadly ideas of history.
As a philosopher and more specifically a determinist I find this quite humorous.
If you have no control or freewill to choose your environment or your genes, how does it matter which has more of an effect when neither allow freewill?
Environment/nurture is not merely limited to where you live. It would include all life experience. Everything perceived through senses. No one will ever experience the world from the same point of view. (which is why identical twins who share the same environment are not identical personalities). Life experience is perhaps a better word for people to use as environment is misleading in this classic argument.
As a fetus you start with how you were nurtured (did your mother take her vitamins or take illicit drugs) and your genetics which can have an affect on numerous things from intelligence to personality traits (I seem to remember the discovery of a gene for humor).
Seeing as how you choose neither of those you ultimately have no control, only the illusion of control or true choice. You will make choices in life, but the reason you made those choices can always be explained by the aforementioned catalysts.
Nothing we do is random. We have a much better understanding of how and why people act in a certain way. If things were random, we would still not have freewill.
If I haven't made this idea transparent enough at this point then please tell me and I will attempt to clarify. I know it is difficult to accept.
Here is a link to the specific scene that exemplifies, in part at least, what I'm trying to explain.
http://www.youtube.com/watch?v=K6Wvpw0sDl0
"Whether by accident or design, there's not a thing we can do about it."
If we ran our nature/nurture behavioral studies on the bacterium we would obviously find out that nurture had almost no role. If bacterium can produce genetic variations which favor one behavior rather than another as it evolves, then there might be behavioral differences that we would attribute to what? Nurture? If an individual bacterium had some completely non-sapient mechanistic deterministic way of learning which behaviors were working in it's current environment, and modified it behaviors accordingly, would that be nature or nurture? Isn't the behavior entirely genetically determined by the form called a bacterium, even though different bacteria may have "learned" different things?
What's nurture? Isn't "free will" the elephant under the table in this argument, and isn't "free will" entirely an artifact of our brain creating our experience, and not something that exists in the reality of hard science?
In the end, I think the world would be better off if more of us were in the nurture camp. Polls suggest most Europeans think this way about issues of poverty and inequality, leading to far more progressive policies than we have here. But again, this is primarily because of my moral values.
As lay people, we should not delude ourselves by claiming that we can interpret this data w/out bias.
I too am liberal leaning, but am not threatened by the notion of genetics playing a role in determining life outcomes. I thought the denial of science for the sake of politics was exclusively a conservative trait. Here, I am proven wrong, and once again, the truth lies somewhere in the middle and incorporates parts of both ends.
It does not challenge my liberal idealogy that persons are predetermined to exhibit certain traits based on their gene pool. It does nothing to discredit the notion that life experiences and access to education, positive role models, and financial means serves to move each person on a more positive route. Even Republicans do all they can to supply their children with effective education. And if they truly believed that success is predetermined and not based on economic means, they wouldn't scream so loudly when asked to pay their fair share of taxes.
One reason ppl w little or no scientific education or understanding attack science is that science, when it's done correctly, is indifferent to ideology or agenda. Results are results. The evidence is there or it isn't. And from what I read, w only a layperson's understanding of psych & genetics, this study certainly doesn't dispute the importance of environmental factors like socio-economic status. Far from it. You may want to read it again, a little more slowly.
But what if it did? What if massive amounts of scientific research confirmed over & over that genes were more important than environment? You would hold onto an ideology after it was conclusively proven false? If you could deny that evidence, what other evidence would you deny? How would continuing to base political/social systems on ideals that do not reflect reality help anyone?
Ideology is the enemy of reason. It's good that you can recognize your own bias, but you must take the next step and challenge ideology when confronted w conflicting evidence. Otherwise, you will open yourself to dishonesty & denial.
Here is a fact that isn't always known. You can cite examples from other species, but in the decades of attempts to find genes for humans linked to specific personality traits, psychological disorders, IQ, etc., not one specific gene or genetic loci has been conclusively demonstrated to be linked to any of the above, despite thousands of studies and even front page news stories touting preliminary studies that are then refuted or never replicated. This is what we might call an inconvenient truth.
P.S. I have an M.D. if that is important.
Also, having an MD doesn't make you an expert in genetics, though I know many doctors do dabble in certain areas out of their immediate area of expertise -- many of my doctors delight in describing the metabolic pathways taken by various drugs they prescribe for me, but I know none of these doctors has ever worked at a pharmaceutical lab, and none would ever do original research to develop a drug for a specific patient.
Since Adops10 already mentioned a bunch of examples to disprove your claims, I will just mention a couple others: XYY syndrome (can lead to learning difficulties, among others), and Lesch-Nyhan syndrome (causes self-mutilation and head banging behaviors, and is recessive on the X chromosome, so most often suffered by men). Science Friday on NPR did a great piece on Lesch-Nyhan a few years ago.
And if your skin & hair color made your ancestors more fit to survive & reproduce--say, fair skin took in more vitamins from sunlight, fair hair more attractive to the opposite sex--then yes, nurture could play a part in your skin & hair color. But most geneticists think that genetic drift is responsible for changes like that.