Natural selection in our time

By Razib Khan | October 11, 2010 1:35 am

Last month in Nature Reviews Genetics there was a paper, Measuring selection in contemporary human populations, which reviewed data from various surveys in an attempt to adduce the current trajectory of human evolution. The review didn’t find anything revolutionary, but it was interesting to see where we’re at. If you read this weblog you probably accept a priori that it’s highly unlikely that evolution “has stopped” because infant mortality has declined sharply across developed, and developing, nations. Evolution understood as change in gene frequencies will continue because there will be sample variance in the proportions of given alleles from generation to generation. But more interestingly adaptive evolution driven by change in mean values of heritable phenotypes through natural selection will also continue, assuming:

1) There is variance in reproductive fitness

2) That that variance is correlated with a phenotype

3) That those phenotypes are at all heritable. In other words, phenotypic variation tracks genotypic variation

Obviously there is variance in reproductive fitness. Additionally, most people have the intuition that particular traits are correlated with fecundity, whether it be social-cultural identities, or personality characteristics. The main issue is probably #3. It is a robust finding for example that in developed societies the religious tend to have more children than the irreligious. If there is an innate predisposition to religiosity, and there is some research which suggests modest heritability, then all things being equal the population would presumably be shifting toward greater innate predisposition toward religion as time passes. I do believe religiosity is heritable to some extent. More precisely I think there are particular psychological traits which make supernatural claims more plausible for some than others, and, those traits themselves are partially determined by biology. But obviously even if we think that religious inclination is partially heritable in a biological sense, it is also heritable in the familial sense of values passed from one generation to the next, and in a broader cultural context of norms imposed from on high. In other words, when it comes to these sorts of phenotypic analyses we shouldn’t get too carried away with clean genetic logics. In Shall the Religious Inherit the Earth? Eric Kaufmann notes that it is in the most secular nations that the fertility gap between the religious and irreligious is greatest, and therefore selection for religiosity would be strongest in nations such as Sweden, not Saudi Arabia. But as a practical matter biologically driven shifts in trait value in this case pales in comparison to the effect of strong cultural norms for religiosity.

Below are two of the topline tables which show the traits which are currently subject to natural selection. A + sign indicates that there is natural selection for higher values of the trait, and a – sign the inverse.  An s indicates stabilizing selection, which tells you that median values have higher fitnesses than the extremes. The number of stars is proportional to statistical significance.



Some of this is not surprising. The age of the onset of menarche has been dropping in much of the world. I suspect this is mostly due to better nutrition, but a consequence of this shift is earlier fertility for some females. The authors are nervous about the robust correlation of higher fertility with lower intelligence, but notice that the pattern for wealth and income is different and more complicated. The key is to look at education.  Whether you believe intelligence exists or not in any substantive concrete sense, those who are more intelligent are more likely to have had more education, and there’s a rather common sense reason why investing in more schooling would reduce your fertility: you simply forgo some of your peak reproductive years, especially if you’re female. The higher you go up the educational ladder the stronger the anti-natalist cultural and practical pressures become (the latter is a heavier burden for females because of their biological centrality in child-bearing, but both males and females are subject to the former). As with religion even if the differences have no biological implication because you believe the correlations are spurious or reject the existence of the trait one presumes that parents and subcultures pass on values to offspring. If higher education has anti-natalist correlations we shouldn’t be surprised if subsequent generations turn away from higher education. Their parents were the ones who were more likely to avoid it.

We live in interesting times.

CATEGORIZED UNDER: Biology, Environment, Evolution, Genetics

Comments (25)

  1. Katharine

    I had a snarky comment all ready for this, but I don’t think I can do anything but sob.

    What is it, exactly, that predisposes people to religiosity?

  2. Katharine

    I’ve also kind of thought that if we did some social restructuring – e.g. made it easier for people to both have kids and do what they want to do and, though this is a bigger change, invented artificial wombs or something so everybody could have the opportunity to be pregnant in a sense (the only thing one would technically have to be dependent on either sex for would be a gamete rather than burdening one sex with the apparent pain in the ass that is pregnancy for nine months – and maybe the weirdo far right would back the f*ck off of women as a result since the playing field would be leveled as far as regards reproduction) – the fertility rate among the smart might go up .

    How to engineer a concomitant fertility decrease among the stupid is another thing.

  3. Sandgroper

    Katharine – Genes + environment + culture, like just about every bloody thing else.

    When I was 3 years old I had a lot of trouble reconciling the contemporaneous existence of both God and Santa Claus. By 4 I had figured out that Santa Claus was bullshit peddled to small children by allegedly responsible adults and confided the same to my older sister, who had already figured it out too (but had kept it from me because ‘he’s too young to know’), so we made a pact to keep it a secret from The Adults in case we wouldn’t get any more Christmas presents if we said we knew it was all crap. I never really did buy the God stuff, because the OT/NT conflicts didn’t make sense, and I knew enough biology to know that baleen whales can’t swallow people, but I kept that quiet because I was an obedient little boy who wanted to please the adults who were peddling the bullshit to me.

    Then, weirdly, when I was 11, I became rather zealously and irrationally religious – to get up at 6.00 a.m. on cold winter Sunday mornings, dress yourself in your one and only suit (with short pants) and trudge off alone to Anglican Communion while the rest of your family stay snugly warm and snoozing in bed takes a bit of motivation and dedication. When you are 11, a sip of altar wine on an empty stomach can make you feel as sick as a dog, not to mention being a great way to spread influenza through the congregation, but I persisted. That lasted until I was 13 and realized that I really didn’t believe at all what I had been kidding myself I believed in, and that I was just in it to enjoy the ritual and feeling of community and approval. What finally blew for me was the Bishop was visiting our parish one Sunday, a big fat red haired Englishman in a dress and a big fancy hat, and he farted loudly in the middle of his sermon – the ritual having been destroyed for me by that presumably involuntary act (for which he evinced no remorse), I realized that for me there was nothing behind it – the ritual was the whole sacred deal, and there was nothing more.

    I now understand that ‘tweeners’ are rule-driven and enthusiastic little zealots, which can be useful for some important things like spreading information about public health and safety and similarly useful things, but is a double edged sword when it comes to unhelpful mythology. But that all flies out the window when puberty kicks in and their chemistry goes to hell, and they start growing hair and wanking.

    But I kept that a secret for a while because the church youth group was a good place to meet cute babes, plus I could sneak out the back to smoke cigarettes with my mates – that lasted until I was 15, when I got kicked out of the youth group for sneaking out the back to kiss cute babes and smoke.

    At that point I finally admitted to myself that I was an avowed Atheist and always had been, as long as I had self-awareness, despite all the adults I knew having pushed me to believe all this stuff. I have to conclude that I was surrounded by the environment and had the culture rammed down my throat, but just didn’t have the genes for it.

    In my daughter’s case, I stayed out of it and let her make her own discoveries – I answered questions as honestly and objectively as I could, but I didn’t try to influence her one way or the other. It was interesting that she followed the same trajectory that I did (except for the cigarette smoking and kissing stuff) (I think). The devout little 11 year old Catholic turned into a confirmed Atheist by age 15, but she kept it quiet so as not to piss off the teachers at school, for purely practical and pragmatic reasons. (Where we were living at the time, the Catholics run by far the best schools, and tolerating the religious instruction is a small price to pay for a good education and thus your whole future. The secular schools there are rubbish by comparison. And yes, the Catholics there teach evolution and nothing else.)

    OTOH, my wife is mildy religious in the sense of superstition – she’ll go with anything she thinks might help – Catholicism, Buddhism, Taoism, Confucianism, without actually knowing very much at all about any of the first 3, and progressively growing out of the 4th. An amusing example is that any time we have stayed in a hotel in Thailand while on holiday, she always insists on visiting the spirit house in the grounds of the hotel and bowing to it on the day we leave to fly away, because she heard somewhere that it is a sure-fire way to prevent the plane from crashing.

    Well, it works – so far, the plane has never crashed 🙂

    The final chapter for me was when I took up meditation, as a way of dealing with occupational stress, and after several months of self-disciplined practice I had a ‘spiritual’ experience – I ‘encountered’ myself as a 4 year old boy, and was struck by what a nice little kid I was. We had a dialogue, the 4 year old me and my adult self, and I realized what a nice little boy I had been, and how the environment I had grown up in had tried to fuck with me, and how I had been trying to act out what other people demanded of me for most of my life, instead of being true to my true self. But in reality what meditation did for me was that it enhanced my long term memory and concentration, and there are good scientific reasons for why it does that. But different people get different things out of meditation, so don’t take my word for it – if you do it, you do it to see what you get out of it, and different people get different things. I’m not peddling it, you understand, it’s a very boring and tedious thing to do, but in my case it yielded some benefits I was not expecting.

  4. I find it hard to believe that religiosity is heritable. I’d think what is heritable is a believe in supernatural explanations, the tendency for which might have a neurological basis. But that supernatural belief might be superstition, magic, or spiritualism as well as some religion of one sort or the other. That religions dominate I think is due to social and cultural reasons. (Belonging to a wide-spread cult has benefits.)

  5. bee, i agree. i was going to say that acceptance of supernatural claims is heritable, and then gone on to say that supernatural claims are a necessary precondition of religion, but i thought that that would be longer than it needed to be.

    katherine, re: artificial wombs. i think that would have an immediate effect in that some women who have zero or one would have ~2 children. the main issue is that there are very strong social norms among the educated middle class to not go above two children.

  6. benj

    ” I do believe religiosity is heritable to some extent”

    If this was true, there would be no secular people today. 300 years ago, all or most of Europeans were religious and believers, and now most Europeans are totally secular and irreligious.
    Regarding the fertility of religious people – contrary to what you write, the gap in western countries is real but not very big (around 0.5 or less child per woman usually, for population of European descent if I remember well).
    Furthermore, the “return to Islam” in the muslim world happened conjointly with a sharp decline in fertility – today Egypt, for example, is much more religious than it was 50 years ago, and the fertility rate is much lower – even lower since 2009 than in Israel. Israel being maybe the only country where the gap between religious and irreligious is wide – but in this country even the seculars have a lot of children (relative to western standards).

  7. benj, i’m not a retard. please don’t comment as if i am. it gets on my nerves, understand?

    1) i said it was modestly heritable. that does not mean that i have believe directional selection was always present in its current direction or at all. that’s evident in the rest of the comment by implication. i’ve made the same argument you have before, which is why i stated “we shouldn’t get too carried away with clean genetic logics.”

    2) the absolute gap is not large, but the relative gap is very large. additionally, there are super-fecund sectarian minorities whose TFRs are in the 3-6 range in societies where the median is in the 1.5-2 range (hutterites, amish, laestadian lutherans in finland).

    3) as for the muslim world…i don’t even know if the muslim world is more religious or less religious than it was 50 years ago. it’s elite culture is exoterically religious. but i don’t think that western models are appropriate to most underdeveloped countries any how.

    4) within societies the key is to look at relative differences, not absolute ones.

  8. benj

    I never said you were a retard. I just can’t agree with the idea that religiosity is *biologically* heritable. I don’t like the tendency to use genetics to explain everything (I am not speaking about you in particular).

    You are right that the relative gap is large but the absolute numbers still count because hte small absolute difference means it will take a very long time for this difference to have any effect so it is meaningless in practice.

  9. Katharine

    i think that would have an immediate effect in that some women who have zero or one would have ~2 children. the main issue is that there are very strong social norms among the educated middle class to not go above two children

    Which goes back to the whole issue of relative proportions versus absolute numbers, of course.

    As I said: needs a concomitant decrease in other fertility rates.

  10. I never said you were a retard. I just can’t agree with the idea that religiosity is *biologically* heritable. I don’t like the tendency to use genetics to explain everything (I am not speaking about you in particular).

    who cares if you don’t like the idea? there is a evidence that it is heritable. but the bigger issue is that heritability does not imply positive directional selection as you implied above. you can elide the logical leap you made, but i can read. if you think heritability implies strong selection, you should read population genetics. the rule of thumb is opposite. if a trait is heritable that indicates that it hasn’t been subject to strong directional selection in the past. don’t talk about what you don’t know about.

    (the above comment should stand as an example to the rest of you)

    You are right that the relative gap is large but the absolute numbers still count because hte small absolute difference means it will take a very long time for this difference to have any effect so it is meaningless in practice.

    i wonder if your thought process is too subtle for me to understand.

    let me waste some time quickly and give you the benefit of the doubt. here’s my thought process. assuming two populations which don’t interbreed and nonoverlapping generations, and start off at a 1:1 ratio

    scenario 1:

    population A = 1.5 children per couple
    population B = 2.5 children per couple

    at generation 4 the ratio of A to B 1:4.62

    scenario 2:

    population A = 3 children per couple
    population B = 4 children per couple

    at generation 4 the ratio of A to B is 1:2.37

    if you keep the fertility gap fixed, the difference over the long term in ratio becomes larger as you drop median fertility toward replacement. because of malthusian assumptions evolution is often conceived as relative differences, not absolute ones. that is not always appropriate in a human context, but in developed nations the reproductive output is now as if conditions were malthusian (replacement, or below).

    your next comment better make some sense and not waste my time.

  11. just a minor clarification: low heritability tends to imply high fitness implication of the trait under consideration, and high heritability low fitness implication. that is how it connects to past selection.

  12. Some of the previous exchange of comments could probably have been avoided if you hadn’t used the words ‘I believe…’ and had replaced it by ‘there is evidence..’. In this case, there was no need for it, as there it seems to be quite a lot of data supporting heritability of ‘religiosity’ (but I am not an expert). But anyway, thank you for pointing out this review, which should shut up people still ‘believing’ human evolution has stopped.

  13. miko


    I see where you’re coming from but you don’t understand Razib’s position, I think. When we say a trait is heritable, it doesn’t mean that there are genes for producing that trait. Liking artichokes probably has some degree of heritability if someone checked, it doesn’t mean artichokes have been a selective force in human evolution. There are many psychological characteristics (and I think you would agree that at least some of these can be somewhat heritbale) that might make one more or less disposed toward supernatural belief under particular cultural circumstances. In some circumstances, e.g. farmers in the middle ages, almost everyone tends toward religiousness. In others, e.g. Swedish biologists, very few.

    I don’t buy that we are necessarily tending toward becoming a more religious species because the religious are out-reproduce the non-religious right now. The social factors that are required to produce religious tendencies might also be changing over time, and different social factors might differently interact with different genetic profiles. That is, the genetically same person who tends toward religion now might not tend toward it in the Space Colonies. Religions can change too–what if Lord Xorkon wants population control in a few hundred years?

    This is somewhat inconsistent with the idea that there is a stable “believe in magic” trait, but I don’t think there is much evidence either way. I tend to think there is not a “magic” module but an “agency attribution” module that was useful for predictive reasoning, which somehow got hooked up to the limbic system and reward pathways. Dopamine’s the only thing that could be making those evangelists’ eyes so glassy.

  14. when i use the phrase “i believe” it connotes that though i suspect that X is true, my confidence is low enough that i want to qualify that it’s my considered opinion, and shouldn’t be taken as a background assumption. as for there is ‘evidence,’ there’s evidence for a lot of things. there’s evidence that prayer is efficacious in medical interventions, but that’s probably a statistical artifact. in any case, i actually linked to pointers to the studies.

  15. I don’t buy that we are necessarily tending toward becoming a more religious species because the religious are out-reproduce the non-religious right now.

    we aren’t becoming more religious in developed nations. rather, some (e.g., spain, USA) are arguably getting more secular. the argument is that in the long term increase will outpace defection. i’m moderately skeptical of that model myself because 50 year projections are always dicey in our fast-changing world (though not as much in 1600).

    but, taking an aggregate international sample, the most secular regions, europe and european settled nations, and east asia, have lower fertility. the near future will have more africans and more middle easterners, who are strongly culturally constrained toward religion. this may change at some point.

  16. expeedee

    I think that religion is the oldest form of psychotherapy. It serves the purpose of uniting people into a form of group therapy, with singing serving as a form of breathing exercise, and rituals and mantras serving to focus the mind. Religion helps with stress, offers group solidarity and support, and has many positive effects. It certainly is correlated with fitness as it serves the purpose of strong group selection.

  17. DK

    I just can’t agree with the idea that religiosity is *biologically* heritable.

    How can it be not? Proclivity to religion is some combination of psychological traits. Psychological traits are partially determined by genes. Ergo, religiosity is most likely biologically heritable *to some extent*.

    I don’t like the tendency to use genetics to explain everything

    Whether you like it or not, fact remains that genes are involved in everything animals do.

  18. i still recall circa 2004 a discussion on ikram saed’s weblog where we were having a discussion about transmission of religion from one generation to the next. i offered that there was evidence that religiosity was 50% heritable. saed believed it was 100% environmentalist. as i suggested that it was 50% heritable and he believed it was 0% heritable (in the genetic sense) he surmised that i could be termed a genetic determinist and he was an environmentalist determinist. how’s that for symmetry? the person who accepts the importance of both genes and environment is a genetic determist 🙂

    steven pinker has mentioned this issue as well. and it’s happened to be in my “real life.” it’s like genes are like black ancestry in the american south; one drop is sufficient to overwhelm all other variables and re-categorize.

    as it happens, i think the 50% value is probably on the high side btw.

  19. Katharine

    I wonder if the discipline of neurotheology could offer some information here – e.g. the temporal lobe study at Laurentian University.

  20. Katharine

    Regarding fertility being the measure by which religion shall drive our species to the death of civilization if it’s not dealt with, have we not considered that these offspring of theirs presumably have the ability to think?

    We can perhaps make fellow atheists out of them yet.

  21. have we not considered that these little crotchfruit of theirs are deconvertable?

    …oh god, now i’m gonna hear it….

  22. Katharine

    Oh no no no no.

    Razib, WHO is making these emails to you about my misanthropic propensities? 😀

  23. Chris T

    We can perhaps make fellow atheists out of them yet.

    Atheism and fertility are hardly mutually exclusive.


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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at


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