All Human Behavioral Traits are Heritable
Edit, 1/3/13: [Post updated to reflect additional information provided in the comments. See below and see the comments.]
The time has come for a little reminder of the First Law of behavioral genetics. In my final post of 2012, I will discuss this extremely important law in depth. As we may recall from debates with creationists about the reality of evolution, scientific “laws” are comprehensive facts of nature that have the virtue of being able to be expressed in a few sentences, typically one or two. The three laws of behavioral genetics are no exception:
- First Law. All human behavioral traits are heritable.
- Second Law. The effect of being raised in the same family is smaller than the effect of genes.
- Third Law. A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.
These laws are extremely important to understanding human behavior, particularly human differences, which is precisely what those of us who study HBD, Human BioDiversity investigate. Anyone who seriously considers this subject should be intimately familiar with these laws, the evidence from which they’re derived, and their implications.
To HBD’ers especially, the First Law is of paramount importance, and is often quite overlooked by the very people who stress heredity in explaining human differences.
But, what does this mean? How do we know this? The First Law emerges from studies of twins, studies of adoptees, and (now) sibling genetic similarity studies. In short, when you look at people’s behavior, virtually without exception (with a few important ones which I’ll soon discuss), you find some effect of the genes on these traits. That is, identical twins reared apart are similar; identical twins are more similar than fraternal twins; biological siblings are more similar than adoptive siblings; siblings who share more DNA are more similar than those who share less. This is not just in the most talked about trait in HBD, IQ, nor is it just in broad personality traits (whether it be the on the Big Five, on the HEXACO, or on whatever model you want to use), but in one’s religious inclination, one’s political views (including one’s thoughts on topics such as abortion, the death penalty, or welfare), career/educational interests, even one’s degree of thinness/fatness (one’s body mass index, or BMI, which is actually 80% heritable, as heritable as height or IQ). This includes even the things that “really matter”: life outcomes, such as one’s chances of completing high school and college; one’s chances of divorcing or getting into trouble with the law; even one’s lifetime earnings. And ask any adoptee that has found their birth family as an adult about the heritability of all manner of odd quirks. Edit: [See this breakdown of the heritability of political stances on specific political issues, adapted from Hatemi & McDermott, 2012, h/t Breviosity.]
How could this be, you may ask? How could such complex and highly specific things be encoded in the DNA and express themselves despite decades of upbringing and childhood experiences? For one, heritability is only probabilistic, not absolute. Few traits are 100% heritable. As well, heritability, as estimated by these studies (even the new genomic ones) can only tell us that traits are correlated with genes. These are points to which I’ll soon return. But, to answer the earlier question, how could these traits be specified by the genes, it is helpful to think how the genes are involved in crafting the brain. First of all, over half of all the genes are expressed primarily or exclusively in the brain. The great wealth of genetic variation that exists in the human species is likely to most manifest itself in one’s intelligence and behaviors. But what does this mean?
The human brain is an extremely sophisticated and incredibly powerful organic computer, unparalleled in ability by anything yet created by man. Yet, like any computer, it is limited in its range of output to given inputs. How these outputs are produced from given inputs depends on the details of its construction, and those details are encoded in the genes. Sure, the brain does respond to environmental input when wiring itself up, and indeed, normal development is impossible without a certain range of environmental input. The development of the human brain is a complex interplay with genes and environment, with each interacting to produce the final product. One would imagine then that this may render heritability minimal, even meaningless; yet, the evidence shows that this is not the case. The reason is that, quite often, these complex interactions proceed in a rather “deterministic” way. There are only a restricted set of possible outcomes, given the initial conditions. These initial conditions being partly specified by heredity. The lateralization of the brain may be one such example. Both hemispheres of the brain are fairly similar. However, human cognitive abilities appear to be highly lateralized, with certain abilities residing in certain hemispheres. It is possible that each side begins roughly equivalent to the other side, but with small biases that become magnified over time. The complex series of feedback loops, while often involving extensive environmental interaction, nonetheless settle on pre-set results as development unfolds. Those of us with receding hairlines (such as myself, unfortunately) should have an idea of how that could work.
But, it’s important to understand the meaning of the term heritability. Heritability is the degree of variation in a studied population that can be attributed to genetic variation in that population. The cause is the variance in question is always due to some genetic difference, but it doesn’t tell you how direct such genetic influence is. It doesn’t distinguish between [gene x + gene y + gene z → A trait] and [x + y + z gene leads to B, which leads to C, which has an effect on D, giving you trait E]. For example, in a population study, a penchant for wearing high-heeled shoes would be highly heritable, but not necessarily because of genes that influence this, but because this is something highly common among those of the sex female. (Of course, a within-sex behavioral genetic study would find that it is heritable in the usually considered sense.)
As well, heritability, as discovered in behavioral genetic studies, only refers to the degree that traits in question correlate with genes with respect to the environmental variation in the study. A study with a restricted range of environmental variation, such as among WEIRD people, will derive a higher heritability estimate than one with a more mixed set of subjects (of course, HBD’ers would argue, correctly, that such samples also have smaller genetic variation as well). The best example of this is height. While height is about 80% heritable in the modern Western world, it is considerably less heritable in the developing world, since factors that influence final height, such as childhood nutrition, are far more variable there than in the West (the heritability of between-group differences, the foundation of HBD, is a point I’ll discuss shortly).
This background environmental variation has big implications for phenotype expression. Just as a seed planted in one type of soil will yield different quality of fruit than if planted in another type of soil, broad environmental changes can lead to large differences in behavioral traits even in the absence of genetic change. This is a sticking point in discussion of heritability of behavioral traits. Certain commenters (such as recently Dennis Mangan and Heartiste) have noted that there have been distinct changes in behavior over the last century, particularly, such as a marked increase in single motherhood, something which is highly heritable today. Like with secular increases in average height, average BMI, and average IQ, this sticking point ignores the fact that the general environment has changed. In the case of single motherhood and divorce, social mores have changed to make this more acceptable, so, those with genotypes more susceptible to exhibiting this behavior have done so, hence, a change in phenotypes.
That said, as racial differences in IQ demonstrate, there is only so much of a difference environmental changes can make. It’s not exactly a straightforward matter to engineer the environment in such a way to get exactly the phenotypes you want with a given set of genotypes. Certain behavioral traits are simply to be accepted as inevitable and dealt with accordingly.
So, how iron-clad is the First Law? Clearly, not all traits are heritable, right? Right. However, there are only a distinct set of exceptions. Traits that are dependent on content aren’t heritable at all. These include what language you speak, in which particular church you worship, what specific political party you identify. However, the degree and manner to which one interacts with these things are very heritable: how proficient you are with language, how church-going you are, how liberal or conservative. In short, genes can’t specify the content, but they can strongly affect how you interact with that content.
As well, traits that result from physical damage aren’t heritable. A person who stutters because of a brain injury doesn’t do so because this was encoded in his genes.
Chromosomal abnormalities, like Down’s Syndrome, also fall into this category. These are merely flukes, and are hence not heritable.
Environmental toxins also lead to effects that aren’t heritable at all. The most well-known of these is lead poisoning, which is known to negatively affect IQ.
As well, recent scientific research has discovered that microorganisms, particularly those that take up residence in the brain, can seriously affect behavior. The poster child for this, the protozoan Toxoplasma gondii, which leads to what’s colloquially known as “crazy cat-lady syndrome”, or toxoplasmosis, occurs when the protozoan infects the brain of a human host, often leading a variety of behavioral changes. Other microorganisms appear to alter our behavior through similar mechanisms. Theoretically, these type of infections would lead to behavioral traits that aren’t heritable at all, as Greg Cochran suspects is the case with male homosexuality. (However, it’s worth noting that even in these cases, differences in behavioral traits may still be heritable, because the infection may only manifest as behavioral changes in individuals with certain genotypes.)
With those exceptions noted, all human behavioral differences show some genetic component, typically a rather large genetic component:
Here it’s worth discussing the probabilistic nature of heritability. Few traits are 100% heritable. As Satoshi Kanazawa noted in his latest post, traits that are simple Mendelian or nearly so have a heritability of 1.0:
Some genetic diseases like Huntington’s disease have heritability of 1.0; genes entirely determine whether or not you will get Huntington’s disease. If you have the affected gene for the disease, it does not matter at all how you live your life or what your environment is; you will develop the disease. One’s natural eye color or natural hair color also has heritability of 1.0. So does one’s blood type.
On the other hand, heritability of 0 means that genes have absolutely no influence on a given trait, and the environment completely determines whether or not someone has the trait. No human traits have heritability of 0; genes partially influence all human traits to some degree. (This is known as Turkheimer’s first law of behavior genetics.)
A heritability between 0 and 1.0 indicate that genes only partially predict behavioral traits. Often, this is a sticking point in discussion about heritability, since people seem to be often caught up in a false dichotomy between something being either 0% heritable (meaning not genetically influenced at all) or 100% heritable (meaning genetically determined). Random factors, not the least of which being developmental noise (a phenomenon which appears to be incredibly important), adds a degree of variation in genetic expression. A positive but less than perfect heritability indicates that when you hold genes constant, as you do with identical twins (almost) you still find some variance (but far less so than less genetically related individuals, which is the whole point).
(The sticking point about partial heritabilities isn’t an objective criticism, but is really about changeability. Since we can’t do anything about our genes, people view heritability as a death sentence or sorts—a condemnation to be a certain way. But a less than total heritability appears to leave room for external manipulation, giving hope to these wishes. This is a rather naive view for a variety of reasons, but has no bearing on the reality of heritability of behavioral traits.)
Edit: [In the comments, Henry Harpending has highlighted the work of Peter Visscher and associates. Using population-wide (UK and Norway) genomic analysis from large representative samples, they have affirmed the high heritability of behavioral and physical traits, including IQ and height (traits which are remarkably similar in terms of their genetic expression and heritability). This refutes a common criticism leveled at traditional behavioral genetic studies: that gene-environment correlation is responsible for the observed variation, hence high heritability (e.g., identical (MZ) twins are more similar than fraternal (DZ) twins because people expect MZ twins to be more similar and/or treat MZ twins more similarly than they do DZ twins). These studies get around that problem by looking at unrelated individuals and assessing heritability through genetic similarity alone (akin to sibling genetic similarity studies mentioned above).]
The high and universal heritability of behavioral traits makes standard family and parenting studies worthless. Obviously, any study that tries to examine the effect of parenting and the family environment by studying children and their biological parents will be confounded by heredity. Indeed, I often purposely use the term heredity as opposed to “genetics” or “DNA” to illustrate where the genes come from. There is a reason that the apple doesn’t fall far from the tree. Indeed, The First Law, coupled with the other two Laws, is precisely why we know parenting doesn’t leave a lasting impact on children.
To briefly recap, the Second Law – on the effect of families, is based on the discovery made by behavioral geneticists that the effect of growing up in the same home on behavioral traits is zero. This includes all the traits I previously discussed, including IQ and BMI. These include those “meaningful” life outcome traits, including the money you end up making. In all these traits, children growing up in the same home are no more similar than those growing up in different homes. Identical twins are ~50% similar whether raised together or apart. Adopted siblings are no more similar than random strangers. This isn’t just on paper and pencil intelligence and personality tests, but is visible in deep structural and operational properties of the brain, as seen in this newly released study by Stokes, Turkheimer, et al. All the popular ideas about the importance of parenting collapse when faced with this reality. Contrary to the enshrined conventional wisdom, even in the HBD blogosphere, parenting is simply not that important in the big picture.
(This, of course, is apparently past a certain baseline. Human children appear to need a certain minimum level of support and emotional warmth for normal development, to which Romanian orphanages attest. However, beyond this baseline, extra parenting effort, past what children need to get by, is simply ineffective.)
The Third Law – a substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families is essentially the left-over variance when heredity and the family environment are accounted for. This value includes measurement error [Edit: see also Staffan's comment below]. It would also include the effects of pathogenic infections or traumatic injury. This value shows that there is considerable variation in behavioral traits even between identical twins raised together. Judith Rich Harris devoted her book No Two Alike to finding an explanation for this unexplained variance. She posits that humans are born with a personality modification system (which coins the “status system”) that molds our behavior to fit the social niche where we are best able to compete. Since twins, who begin life differently thanks to developmental noise, enter slightly (or sometimes significantly) different niches, they can end up being quite different. In the future, I will make a post examining this leftover variance in detail. For now, I will say that, for a variety of reasons – not the least being the aforementioned developmental noise – non-genetic variance doesn’t necessarily equate to changeability, a non-genetic effect can be just as “inborn” as a genetic one.
The First Law, the reality that all human behavioral traits are heritable, has deep implications for HBD as well. For one, HBD’ers often focus on fairly simplistic differences between racial groups, such as average IQ or average future-time orientation. However, since all human behavioral traits are heritable, all differences between human groups could, and almost certainly do, have genetic roots. Indeed, this reality makes it impossible to state that “culture” (meaning non-genetic factors) alone is responsible for any group-wide difference. Indeed, one can only make such an assertion if either:
- There has been insufficient time for the degree of change to be explained by genetics (as with secular increases in height or BMI).
- The two populations are genetically identical (possible to declare only if you know that there were no sorting mechanism involved in separating the two populations, as well as no subsequent admixture)
These are usually not the case. Indeed, some researchers in the matter, such as Peter Frost, Greg Cochran and Henry Harpending, and HBD Chick (and of course, myself) pursue this avenue. HBD Chick especially has been prodigious in trying to explain human cultural differences in terms of biology, including the most complex aspects such as politics and ideology. The First Law leads one to be “hereditarian“, and realize that genetics may be involved in explaining even the most seemingly benign quirks that differ between human groups.
Sure, a critic might say that there is no simply conclusive way to determine the degree that heredity plays a role in between group differences as there are ways to determine differences between individuals within a group (yet). But such criticism misses the point. Since we can rarely or never a priori rule out heredity as being responsible for group differences (and indeed, evidence points to heredity typically being involved), it makes sense to research how heritable differences could be behind such group differences.
As political and religious attitudes are heritable, genetics are likely involved in explaining the persistent differences between people within a race within a country, such as between American Whites in different parts of the country. Often, why Whites in different parts in America vote the way they do (instead as some sort of monolithic racial block) seems to dumbfound many commenters. Yet, American Whites seem to have distinct genetic heritage, even among Britons, hailing from different parts of the British Isles and then subject to different selective pressures here in the U.S. In the future, I will publish a post detailing the historical settlement patterns discussed in David Hackett Fischer’s Albion’s Seed and the implications for modern Americans. The reality of the situation is that the persistent and often highly contentious differences between White Americans in different parts of the country, differences that often make it seem like we’re from completely different countries (or sometimes different planets) stem from the fact that American Whites in different regions have different mindsets, and this partly because they have different genes.
In the end, exploring the extent of how heredity drives human behavior and how differences in genetic inheritance explain the vast differences that exist being humans – including between individuals and between groups – will be an active area of research for well into the future (with many such research projects already well underway), and will yield many new discoveries for decades to come (with many projects and belying any notion we’re coming to the end of knowledge in this topic). This is why understanding heritability and the nature of heredity are so key.