Sources of Human Diversity

by Kevin Currie-Knight

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People differ in all sorts of ways. Most of our differences generally go unnoticed when we interact. For example, with regard to a store clerk yesterday, I didn’t notice what her eye color or dominant hand is. But other differences, as the saying goes, make a difference. Historically and into the present, we have put great importance on race, ethnicity, religion, and sexual orientation. What I want to do here is talk a bit about what makes people diverse and why this matters.

The reality of human diversity owes roughly to four factors: biology, environment, culture, and individual choice. Let’s survey each of these.

First and likely the most controversial: Biology. When we are born, we get half of our genes and chromosomes from our mother and half from our father. These combine in a unique way to produce an entirely unique individual. Genes are essentially the construction plan for how our bodies turn out. Of course, this also depends on the environment in which our bodies develop.

It’s not just our bodies that owe something to genetics, but also our behavioral traits. Behavioral Genetics has shown that everything from how optimistic or pessimistic we are to whether we gravitate to religion to how susceptible we are to depression owe at least something to our genetics. If you are skeptical, think of it this way: Our brains help determine our behavior, and our genes play a part in developing our brains.

Of course, a lot of things go into determining our behavioral traits, not just genes, but the rest of the variables we talked about: environment, culture, and individual choice. Therefore, for any particular trait, behavioral geneticists try to determine how much of the trait is genetic and how much is not.

For instance, as of 2021, most scientists put the estimate for the genetic contribution to Attention Deficit Hyperactivity Disorder at 74%, or in scientific terminology, it has a heritability of .74. Journalists might be tempted to write headlines like “74% of your ADHD is in your genes,” but that’s not quite what the figure means.

What scientists do is to test large samples of the population. They figure out how many people – say out of 100 – have ADHD, and then they look to see if the people with ADHD in that sample share certain genetic markers in common. To say that ADHD is 74% heritable means that our of all the people in a population who have ADHD, 74% share certain genetic markers. Or as the psychologist Scott Barry Kaufman says in his book Ungifted: “Heritability estimates are about understanding sources of similarities and differences in traits between members of a particular population. The results apply only to that population. The purpose is not to determine how much any particular individual’s traits are due to his or her genes or his or her environment.”

I mentioned that biology might be the most controversial of the things we talk about, and there is a reason. Until somewhat recently, science has had a fairly bad track record when it comes to using biology to justify racism, sexism, and all sorts of hierarchal categories that rank people by worth. Eugenics used the biology of the time to rank people into genetically fit and unfit, superior and inferior “races,” and was even used to determine who could enter the country or who should be sterilized so that they do not bear “genetically unfit” children. Scientists are a lot more cautious when they talk about biology now, but this is why some are reluctant to talk about the biological roots of diversity.

Let’s move to the environment, by which we mean our physical surroundings. As mentioned, physical and behavioral traits generally have some genetics to them, but they also have environmental aspects. To take an easy one, even identical twins – the only people who share the exact same genetics in common – will often grow to be different heights depending on the nutrition they get and the environment in which they grow up. Or the same identical twins may have the genetic markers that would predict ADHD, but environment – everything from your nutrition to your teachers to your home – plays a significant part.

Next, there is culture, which is our social environment. Anthropologist Clifford Geertz calls culture “webs of significance” spun by networked human beings. What religion people follow, what language you speak, the types of things you and the people around you do for fun, how you collectively understand your lives: these are aspects of culture. And people aren’t just members of one culture at a time, but of various cultures based on the social groups to which we belong. Your fellow church congregants may have one way of acting, while your friends at the skatepark have different ways. You may change the language you use, speaking one way at home, another way with friends, and yet a third way with co-workers. What you are doing is what we all do: learning to behave differently in different cultural settings.

Lastly, there is individual choice. Biology, environment and even culture are things we tend to think of as acting on us. But we also act back on them, at least on environment and culture. Your culture may influence the religion you feel connected to and how you worship, but that’s also a matter of personal choice. Language works the same way: How people around us talk obviously influences the ways in which we talk, but language changes because individuals decide at various points to say things differently, to go against the cultural grain and thereby, individuals shape culture as much as culture shapes individuals. We can choose to change and modify our environments too: I can rearrange the space around me to better suit me, or choose to find or create a new space. Scientists call this “niche construction,” in which we create new spaces in our environment to better suit us.

These four variables – biology, environment, culture, and individual choice – are rarely isolated. They constantly interact with each other, and just about any type of diversity we see around us is influenced by more than one of them. Take a person’s religion. Believe it or not, behavioral geneticists claim that a person’s religiosity has a genetic component; that certain genes predict at least to some degree whether someone will be religious. Even if so, what religion you choose (or don’t) depends on all three other factors too. Where you are born and what religion or religions are dominant is both an environmental and cultural factor that surely influences what religion you will be part of (or whether you will be a part of one). But even with all of that, you are still a unique individual and can decide to turn against all of those factors.

The last thing to note is that there are often heated cultural discussions over what factors are responsible for what kinds of diversity. Are there biological differences between men and women, or are those differences purely because we acculturate girls and boys differently? Is race or gender biological or what we might call a social construction? Are homosexuality or bisexuality genetic, the product of individual choice, or something else? The stories we tell about what produces different types of diversity are important because they can affect how we think about that diversity and possibly how to address inequities it might produce. For instance, cultures have grappled with how to treat gays, lesbians, bisexuals, and trans people, and one piece of this is figuring out whether these things have biological roots or not. Some say that if being gay or trans has a biological origin, we cannot and should not treat them as moral weaknesses or choices people should learn to avoid. Others say that telling biological stories about homosexuality or being trans lays the groundwork for treating them as medical conditions to be cured. Remember what I said about Biology’s unfortunate legacy with issues of diversity.

Another example: Is race biological, or is it a social construct? Our answer to this is important largely because if race is something we’ve socially and culturally constructed – if it has no legitimate biological basis – then we can try to “un-construct” it or construct it in less harmful ways. But, if race does have a biological basis, then we’d have to treat it as more real, and we’d wrestle with the thought that the racial differences that have given rise to racism are here to stay.

Of course, there is a difference between the sources of diversity and what we make diversity mean. For instance, it can be true that skin color or other physical features we use to construct racial boxes are genetic without the racial boxes – let alone whatever significance we think they have – being genetic too. And it doesn’t seem clear that we need to decide whether being gay is genetic or a choice to determine whether we should treat gay people with respect and acceptance. We’ll talk about all of that, though, in another essay. Suffice it to say that all the diversity we see between humans has a source – really, multiple sources – and while we don’t always handle diversity well, it can be important to identify where it comes from.

26 comments

  1. “The last thing to note is that there are often heated cultural discussions over what factors are responsible for what kinds of diversity. Are there biological differences between men and women, or are those differences purely because we acculturate girls and boys differently?”

    = = = = = =

    Hopefully, this is not a serious question. We can tell males from females in skeletons from thousands of years ago.

    Reproductive classes — which is what male/female are — are a part of nature. That we acculturate the two reproductive classes differently is an entirely separate matter and irrelevant to the facts of mammalian reproduction.

    1. Of course only fools who can’t believe their own eyes would deny biological differences between male and female and our current understanding of hormones and brain physiology only reinforces this fact. Still is should be noted that it takes highly trained and experienced experts to differentiate the sex differences of skeletal remains (as is does between the “races”.) Perhaps this should metaphorically caution us as to actually how much the sexes would actually differ behaviorally if not for aculturation. After all the very nature of diversity/variation is a spectrum.

  2. I must admit to finding the fixation on diversity bizarre and to a great extent, unhelpful, especially now. But I know that you teach in an Ed School, so the topic is unavoidable. Still, it’s a strange obsession for an Ed program imho.

    1. I share some of that concern. Sometimes, when I teach this Intro to Diversity Course, I get a sense – and students sometimes let me know – that they think we are making mountains out of molehills. Some of the students who say this are from the very groups those who are concerned with diversity issues seem to be most concerned about protecting. Hard to come away from some of that with an idea that this doesn’t just become another variant of the White Man’s Burden.

      But I think there is at least a mild defense I find convincing, and I relay it to students. Public schools differ from most every other institution in civic life in the following ways: it is an institution that, barring small pools of school choice, every kid must go through, and (b) barring those small pools of school choice, no ‘customer’ can leave. That means that teachers will quite often be exposed to very diverse students with very diverse backgrounds and needs, and that when they do, those kids are at the mercy of the teacher. Have gay students in your class but belong to a religion that longs to tell them how sinful they are before converting them? Neither you nor they can leave that situation. Think black people are generally slower and less intelligent than white students? A “race realist”? That could make life a bit harder for any black students you have in the room.

      Add to that that historically, the US has not handled diversity in public schooling well at all. From its start, when the accusation (true) was that it forced non-Protestant kids to recite Protestant creed to the fact that we have an almost as-segregated school system now as we did before Brown was handed down (a decision which evoked decades of blowback), to the strange and slow history of special educaiton… it is fair to say that US schools have accomodated diversity only through kicking and screaming until absolutely made to do it.

      So, I do think mountains can be made of molehillls. But I am not ready to say this is a molehill.

    1. I think this goes to my purpose of this piece as Course Notes. I’m not here taking a stand on the actual heritability of ADHD, but pointing out that a good deal of existing scientific literature finds a host of behavioral traits to have a degree of heritability.

      Not that it is relevant to the piece, but I am also skeptical of the surge in ADHD (and other) diagnoses. I think psychiatry is less the kind of science we want to think it is than an attempt to medicalize deviations from statistical norms that always and irreducibly come with value judgments attached to them. ADHD, for instance, firmly has its history rooted in schools. The more important school has been to childhood, the more demand we have for kids to pay attention to stuff they find boring, and the more we get so used to the idea that we label an inability to do so ADHD an refusal to do so as ODD.

      I am not what one might call a psychiatry skeptic so much I wish people would appreciate the social and normative nature of psychiatry.

      1. And it usually the boys who are pigeonholed the most, for some inexplicable reason – biological perhaps? From roaches to humans, males have higher metabolisms, this might be a factor. One speculation is that this hyperactivity may have been very adaptive during our evolution for reasons of continually exploring and moving to new lands, thus it was never lost from the gene pool and is now under certain modern situations maladaptive.

        This points to the extremely important survivability factor governing all life, diversity/variability. Unfortunately this bedrock of Darwinian evolution is too often overlooked when considering human affairs.

  3. What a marvellously politically correct essay this is. Let us all celebrate diversity and sing kumbaya!

    Who could possibly argue with it? Let us all be woke and as inclusive as possible.

    And yet in America’s humiliatingly incompetent debacle in Afghanistan we are presented with something else that illuminates the matter in an entirely different way. On the one side we have vast power, numbers and technology. On the other side we have a small, poorly equipped rag-tag army. And guess which side won?

    The more purposive, determined and united side won. Purpose, determination and a united vision create enormous strength, to be used for good or bad.

    My point is that you are talking about diversity of presentation, as if that is what matters. But what really makes us effective as a human species is the way we unite in a purposive, determined way to create a deeply enmeshed society that is highly collaborative and specialized. This is what has created modern civilization and culture. It is the foundational layer upon which diversity rests.

    I said above that purpose, determination and a united vision create enormous strength, to be used for good or bad. Which it is used for depends on possessing a shared vision of good, not a diverse vision of good where no-one can agree on whose vision of good prevails.

    Unity of purpose and values, and not diversity of presentation, is what makes all the difference.

    1. It is hard to know how to respond to this given that the lion’s share is responding to nothing I wrote. It seems like your problem with the piece (which is a transcript of some course notes, so it is not meant per se to argue a particular point) is that I didn’t write about Afghanistan and the thing you wanted me to write about. I don’t take requests.

      “Who could possibly argue with it? Let us all be woke and as inclusive as possible.”

      The first sentence here is interesting in light especially that you do go on to argue against what it is you think i’ve said, particularly about whether diversity of presentation is at all important.

      “My point is that you are talking about diversity of presentation, as if that is what matters. But what really makes us effective as a human species is the way we unite in a purposive, determined way to create a deeply enmeshed society that is highly collaborative and specialized”

      It seems like you are proceeding under a strange assumption that only one thing can matter at a time, and that talk of what matters must be talk of what matters irrespective of context (such that it can’t be that x matters in x context and y in y context). I guess I don’t share such a view. It seems like an utterly false choice to me.

  4. I am not sure I understand the point of the article.

    Your point seems to be that we need to craft very carefully “the stories we tell” (as you put it) about the nature and causes of diversity because these stories affect not only “how we think about […] diversity” but also how we might want to deal with the “inequities” which that diversity might produce.

    The moral contortions could, I suggest, be dispensed with if we simply put the focus on getting things right: on rational and scientifically-informed discussion. (Good faith is also necessary of course.)

    “The last thing to note is that there are often heated cultural discussions over what factors are responsible for what kinds of diversity.”

    I am not sure what is meant by the phrase “cultural discussions”. Ultimately what matters is the extent to which *any* discussion of these issues (whether it is “cultural” or not, or heated or not) is scientifically informed and conducted in good faith.

    I might also add that, like Dan, I was startled by the suggestion that the differences between men and women may be entirely due to acculturation!

  5. From a genetic point of view, DNA nucleotide diversity between individual human beings is 0.1% In other words, we are each 99.9% identical to any other human being we meet. In trying to know about ourselves, it thus makes more sense to talk more about human similarity than it does to focus on our diversity. Black, white, red, yellow, brown, cis, trans, man, woman, gay, straight are all of relatively trivial import when it comes to understanding human nature.

    Yet our ruggedly individualistic American society promotes a stridently individualistic ethic that encourages us to spend much mental energy celebrating and indeed perseverating on our own unique individual nature rather than our commonality. While there is certainly value in this given the sources of our diversity outline in the article, the extent to which society spends time discussion various different individual identities and the need to accept and accommodate such difference is our of proportion to our actual diversity. It also has a very dark side.

    Society is predicated on people coming together for mutual benefit and acknowledging a basic commonality in order to construct society. Creating a better society essentially needs a frame of mind in which we acknowledge things like “ALL men (and women) are create equal in the eyes of God” and ” WE the people” and “promote a perfect UNION” and “provide for the COMMON defense” and “promote the GENERAL welfare” and provide “liberty and justice for ALL.”

    This is the paradox. The more we discuss our individuality, the less we come together, the further we drift apart, and the more we permit of bigotry/sexism/xenophobia/etc.. The more we discuss our commonality, the more we construct a better union of individuals. The best way to fight racism or sexism or any other sort similar ism based on individual differences is to stop talking about race or sex or any other sort of difference that exists between humans and rather focus more on our common human identity and human experience.

  6. Can we at least mention diversity of view points? Even if you do not think it deserves to be treated as important for diversity it is at least arguably important in many contexts.

    Is it allowed to say some cultures are better than others at least in some respects? Is it fair to say people can and should reject their culture? Or is culture to be treated more like genetic makeup something we shouldn’t pass any judgment on? That I think that can be the most controversial discussion.

    Is it wise to try to assign the cause of inequities to one of these 4 factors? How is it that sibling birth order tends to have such a significant impact in outcome? The same parents and so it would seem there is no genetic benefit. Same family and culture. Is it that slight bit more attention the older sibling gets before the other sibling is born the cause of this disparity or the feeling of responsibility for a younger sibling? If such individual situations make such a large impact does it make sense to think we can really adjust reality to create equal outcomes across any groups at all? Are there problem with trying to force equal outcomes?

  7. “Another example: Is race biological, or is it a social construct? Our answer to this is important largely because if race is something we’ve socially and culturally constructed – if it has no legitimate biological basis – then we can try to “un-construct” it or construct it in less harmful ways. But, if race does have a biological basis, then we’d have to treat it as more real.”

    = = = = = =

    Are cultural realities less real than natural realities? There seems to be a pervasive belief in popular culture (or perhaps academic culture) that social = socially constructed = chimerical, but it is easy to point to plenty of things that are ‘social’ but hardly less real nor less malleable than trees, rocks, etc. In fact, in a way social ‘constructs’ are *less* malleable than for example biological structures, precisely because we have ‘built’ them and due to this have a hold on us that has, for example, allowed languages and cultures to persist with minimal changes for centuries, and in many cases even to persist in their identity despite such changes – the genetic bases of many European cultures have as far as I know changed over the last few millennia, yet culturally they are regarded as the ‘same’. Something similar could probably be said of most contemporary languages.

    Here’s a story that might be relevant: I used to watch a lot of historical dramas on TV when I lived some years ago in China (these dramas are a big staple of contemporary Chinese television programming; also, my wife liked them a lot). Something that I found really amusing and rather disorienting was that all the characters spoke in utterly contemporary Mandarin, even if the drama was set in, say, the Tang dynasty (i.e around 900 AD), when Mandarin didn’t even exist. That is, there was no attempt to ‘archaize’ the language at all, as we often see in English-language productions. The effect on me was to make these shows appear to me to be comedies, like the British Blackadder series. But I don’t think the Chinese audiences saw them in this way at all. For them, Chinese is Chinese, it is the ’same’, even though in actuality spoken Chinese from the Tang dynasty would likely be incomprehensible to a contemporary Chinese person.

    The point I’m trying to make is that is precisely because certain things like race, ethnicity and culture are socially constructed, that they are so resistant to change, and that our reluctance to recognize this makes us project the basis of this into the natural world. On the other hand, it is comparatively easy to change the value that natural things have for us almost by fiat (the contemporary controversies surrounding transsexuality are a good example.)

    1. Paul,

      I agree with the point you are making. I guess I did these Course Notes right, because a few folks here have confused my raising x as my arguing for x. Really, what I’m doing is surveying the landscape of current discourse. I’m saying that many social constructivists DO think that the more we discover x to be a social construction, the more empowerment this gives us to try and change it.

      But I will say that there is, I think, a cultural danger in downplalying the degree to which certain categorizations are culturally contingent. Race is maybe the easiest to argue. So many folks end up treating race as if it – and really, whatever categories WE happen to use here and now – just ARE the natural categories. He is black, I am white, and that says enough about us that just saying it contributes some useful info. But when we realize the degree to which these categories are cultural – they change with time and place – and the degree to which categories we think are well established were wholly political inventions – some quite recent – I think that does something to our sense of what we mean when we say x is ‘real.’ Maybe not whether it is real, but how it is AND IS NOT real.

      1. kevinck
        I share some of the concerns that Mark and Dan expressed. And I think a prologue explaining this as course notes, and your intent to sue them would have helped.

        That said, I also note that the very idea of such discussions of diversity, especially in the classroom, will of course annoy, perhaps even anger, some, however phrased. As if we should all belong to the same church, the same culture, share the same values, accept the same community standards. I on the contrary believe diversity can be a benefit to many disparate communities sharing the same social space, and tolerance of differences a necessity in a liberal state with democratic aspirations. The probable alternatives are profoundly troubling, dangerous, and potentially violent, no matter how well intentioned. So it may only be a matter of how to discuss the issue, not whether or not.

        1. EJ, Course Notes is a genre category here at EA. There is a separate tab for it. It is not customary for the pieces there to self identify as such.

  8. Thanks for posting this, Kevin. I look for behavioral genetics to raise some eyebrows in the years to come. Many people in our culture, and in education, still largely adhere to a “blank slate” view of human nature. Robert Plomin talks about when he was in graduate school in the 70s, it was still thought that schizophrenia, for example, was the result of what your mother did to you. Through twin studies and adoption studies people like Plomin showed how many traits (IQ, extroversion, mental illness) had a strong genetic component. Since population genetics is continually getting cheaper to do and more robust, I look for larger and larger GWAS studies that will help to identify the genetic markers involved in particular traits

    Some of these discoveries, as I say, may lead to dismay. In an interesting editorial in the NYT, population geneticist, David Reich, attempts to caution people that science may move in uncomfortable directions and that folks would be wise to be prepared
    https://www.nytimes.com/2018/03/23/opinion/sunday/genetics-race.html

  9. Kevin,

    “To say that ADHD is 74% heritable means that our of all the people in a population who have ADHD, 74% share certain genetic markers. Or as the psychologist Scott Barry Kaufman says in his book Ungifted: “Heritability estimates are about understanding sources of similarities and differences in traits between members of a particular population. The results apply only to that population. The purpose is not to determine how much any particular individual’s traits are due to his or her genes or his or her environment.”

    Exactly, which reminds me that also, like in the following study, genetic markers don’t say if the genes are good or bad. The genes in question here are most likely associate with all kinds of positive or useful behaviors (individually and as a social species), and the fact that they also correlate with behaviors that are considered undesirable isn’t more an indicator of rising genetic dysfunction than it is an indicator that the behaviors in question are a reaction to (relatively dysfunctional) social or cultural changes.

    = = =

    https://www.sciencedaily.com/releases/2021/08/210826111741.htm

    “An analysis of data from 1.5 million people has identified 579 locations in the genome associated with a predisposition to different behaviors and disorders related to self-regulation, including addiction and child behavioral problems”

    “With these findings, researchers have constructed a genetic risk score — a number reflecting a person’s overall genetic propensity based on how many risk variants they carry — that predicts a range of behavioral, medical and social outcomes, including education levels, obesity, opioid use disorder, suicide, HIV infections, criminal convictions and unemployment.”

    1. I find the (possible?) assumptions in the last quote very disturbing. We can talk about the canary’s genes and it’s risk score. We can also talk about the environmental factors in the coal mine.

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