Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
Meet Emma Morano of Italy, who was born in 1899 and has attained the status of both “oldest living person” and “only person born in the 19th century”. She turned 117 on November 29.
Click on the screenshot below to go to a video of the world’s oldest woman. Be sure to turn the sound on.
The New York Times also documents her life, which was tough. But thanks to her diet of three eggs per day (according to the video, two raw and one fried), she’s still here. Times writer Elisabetta Polvoledo says this:
I wrote about Ms. Morano two years ago, when she was only 115, and she told me she believed that her secret to longevity was eating three raw eggs a day and remaining single.
Ms. Morano has no doubts about how she made it this long: Her elixir for longevity consists of raw eggs, which she has been eating — three per day [JAC: note disparity between Reuters video and this piece] — since her teens when a doctor recommended them to counter anemia. Assuming she has been true to her word, Ms. Morano would have consumed around 100,000 eggs in her lifetime, give or take a thousand, cholesterol be damned.
She is also convinced that being single for most of her life, after an unhappy marriage that ended in 1938 following the death of an infant son, has kept her kicking. Separation was rare then, and divorce became legal in Italy only in 1970. She said she had plenty of suitors after that, but never chose another partner. “I didn’t want to be dominated by anyone,” she said.
Ms. Morano, who has cut back to two eggs a day, lives a very simple life. She has been homebound for some years, and her diet remains Spartan, if unorthodox: In addition to eggs, she eats bananas and ladyfinger cookies.
Of course when all these people are asked the “secret of longevity”, they say the same thing, which is basically “do what I did.” Still there’s some wisdom in the following:
Ms. Morano’s doctor of nearly two decades, Carlo Bava, said that despite her age, his patient was still in excellent health, and her memory sharp. “She’s in great form,” he said. “And I think she’s happy to have made it to this birthday.”
Diet aside, Dr. Bava said he thought Ms. Morano had lived such a long life because she was cared for. “The secret is in growing old with people who love you, which is different from growing old and being put up with,” he said.
But maybe there’s something to eggs after all. I just remember that the world’s oldest cat, Creme Puff, who lived to be 38 years and 3 days old (!!!!), was fed on a diet of asparagus, coffee with heavy cream, broccoli, and bacon and eggs.
A question I’m always asked in popular lectures on evolution is this: “Are humans still evolving?” The answer I give is “Yes, but we have good evidence for such evolution in only a handful of traits: evolution of earlier reproductive maturity in females, later menopause, and selection for reduced blood pressure and a few other traits related to heart disease.” That is based on longitudinal studies of human health over decades, observing changes in these traits and presumed estimates of the genetic basis of their variation.
Now, however, we can, by DNA sequencing, look at DNA directly, and with some fancy statistical footwork, get an idea of which genes have changed in frequency so fast that they must have been due to positive natural selection. That’s the subject of a new paper in Science by Yair Field et al. (reference and free download below). The authors conclude that several traits, including lactose tolerance, hair and eye color, and parts of the immune system, as well as height, have evolved within the last 2,000 years.
A short primer on how they did it. The authors looked at 3,915 sequenced genomes in the UK population trying to find evidence of recent selection. How does that work? Well, if a rare allele suddenly becomes favorable, it will rise quickly to high frequencies, dragging along with it the DNA in regions adjacent to the selected sites (recombination wouldn’t have time to separate the selected site from adjacent sites on the chromosomes, since recombination is rare between closely spaced bits of DNA). This means that a region of the genome around a recently selected bit of DNA would be genetically depauperate compared to the same stretch of DNA that isn’t around a selected nucleotide. Then, if you look at variable DNA sites in that region, where more than one base pair is segregating at a single site, regions around a selected site would have fewer such “singletons” because they’d be genetically homogenous. And that, in turn, would mean that in regions near selected bits of the DNA, the distance between “singleton” (variable) sites would be larger than in alleles not subject to recent positive selection, since selected forms of genes would be linked to fewer variable singletons.
The authors figured out the statistics of the distribution of the inter-singleton-site distance in the sequenced genomes compared to alternative “alleles”, looking for those regions that showed significantly longer inter-singleton distances around a site that differed between forms of a gene, and thus a site that might have been subject to selection. That indicates that, with all probability, the target site between the singleton DNA bases rose to high frequency fairly recently.
How recently? The authors say that their method is best at detecting selection events (“sweeps”) in the last 2,000 years (about 75 or fewer generations). What did they find? Here’s some figures showing those genes (and traits) whose statistics indicate they were subject to selection during roughly the last two millennia.
Lactase persistence: the gene for digesting lactose, lactase, is usually turned off at weaning. In “pastoral” populations—those that keep sheep, goats or cows and drink their milk—there’s strong selection (a roughly 10% reproductive advantage) to keep the gene turned on, giving you a rich source of nutrients. And that gene shows recent signs of selection (see the longer distance between singletons around the A/A “lactase persistence” allele compared to the alternative heterozygote [G/A] and non-persistent [G/G] form). We knew this in fact from other data over the past decade, but it’s reassuring to see it with these authors’ statistics:
Genes associated with pigmentation. Although the loci for skin color, presumably subject to selection for light skin in northerly climates, didn’t show a significant signal of selection, other genes associated with pigmentation, like blond hair, freckling, blue eyes, and so on, did. The four dark boxes are highly statistically significant, and if you combine all the data, the probability that this association is due to chance is miniscule: 0.000000003. Lighter pigmentation was selectively favored in the British sample. There are several theories of how such selection works, but I won’t go into them now.
Genes for becoming taller. Here’s a plot of the distance between singletons for height increasing alleles (combined across several genes thought to be involved in height) compared to alternative alleles that make you shorter. Although the displacement of the graphs looks minor, the probability that the distributional difference occurred by chance is 0.000000000001.
The Major Histocompatibility region, involved in the immune system, is subject to “balancing selection”, with individuals having two forms of a gene deriving a selective advantage, presumably because they have a more responsive immune system. This, too, inhibits recombination around each selected allele, increasing the inter-singleton distance. This locus also shows highly significant differences from other regions.
Other genes. Finally, there is a host of other genes that give suggestions of having been recently selected: genes or “polygenic” (several-gene) traits. The traits or genes with the purple dots show significantly longer inter-singleton distances, but the authors are a bit less willing (except for height in both sexes) to say these genes and traits were subject to selection because there’s a problem of population structure (non-random mating across the entire group) that could make these differences appear more significant than they are. But here’s the figure just to show you what might have been selected (the pluses and minuses show the direction of evolutionary change).
The upshot: In the last 2,000 years, there’s good evidence that lactase persistence and lighter pigmentation have been positively selected in the British sample of genes, that the MHC complex has been under balancing selection, and that the UK population has been selected to get taller (you can devise your own explanations for that). Along with the putatively selected traits in the figure just above, this considerably expands the list of characters that have been selected in our species in recent centuries. Anybody who claims that human evolution has stopped is simply talking nonsense.
______________
Field, Y., E. A. Boyle, N. Telis, Z. Gao, K. J. Gaulton, D. Golan, L. Yengo, G. Rocheleau, P. Froguel, M. I. McCarthy, and J. K. Pritchard. 2016. Detection of human adaptation during the past 2000 years. Science. Published online, 13 OCT 2016, DOI: 10.1126/science.aag0776
As I’ve written somewhere (but can’t remember where), it always amused me that when I wrote an NIH grant application, I had to specify my “race” (black, Pacific Islander, white, Hispanic, etc.), but then, in the instructions, it said something like “These categories are taken to be social constructs only, and are not biological.”
That statement is palpably false, but comes from the Leftist ideology that if you even talk about races, you’re promoting racism. As an evolutionary biologist interested in human differentiation, I know that the human species isn’t divided into a finite number of well-differentiated genetic groups, but that groups can still be distinguished by combining information from different genes, and that those groups tend to be those that evolved in geographic isolation, telling us something about human evolution. And I’m interested in understanding some of that genetic differentiation, like the processes involved in leading to morphological differentiation in traits like skin color, body configuration, and so on. Is that due to natural selection, sexual selection, or maybe genetic drift? Why do evolutionists pay so much attention to geographic differentiation in animals and plants, but avoid talking about it in Homo sapiens?
The answer, of course, is the ideological view that if you study that kind of differentiation, you’ll be promoting racism. And indeed, that has happened in the past. But I maintain that one can study human geographic variation in a purely evolutionary way, and simply criticize those who try to co-opt that work to set up any kind of racial hierarchy or to promote bigotry. We are, after all, the animal species that most fascinates us.
So when people say “race is a social construct,” they’re simply wrong. The only sense in which they’re right is that the designation of a finite number of easily-distinguished human groups (“races”) is a futile exercise, because we have differentiation within differentiation, making the whole exercise purely subjective. (You can, for example, distinguish subgroups of “Caucasians” within Europe, distinguishing those of Scandinavian from Italian ancestry simply by their genetic differences.)
But that’s not what people mean, I think, by “social construct.” What I think they mean (since they are rarely explicit) is this: “There is no biological difference between human ethnic groups.” That’s just wrong. Or, more plausibly, they mean that groups designated by skin color alone as “races” show no other biological differences that co-segregate with skin color. But that’s not true, either. For one thing, skin color itself is based on genetic differences, ones that (as we’ll see tomorrow) probably evolved by natural selection. And skin color co-segregates with other physical characteristics, as in the group “African Americans.” Finally, there are genetic diseases, like sickle-cell anemia and Tay-Sachs disease, that are more prevalent in some ethnic groups than others, and that is useful biology to know.
I’m writing this because reader Cindy called my attention to an NPR article describing how Brazil is now using skin color to determine who fits into various categories subject to affirmative action boosts. Brazil has “race tribunals” to place people in “racial” groups, and the traits used can include more than just skin color.
The NPR story starts with Lucas Siqueira, who got a coveted government job in Brazil after scoring well on a test and identifying himself as “mixed race.” People looked at his Facebook page, determined he didn’t look “mixed race” but white, and they complained bitterly. The government put his job on hold. The story then gets really bizarre\:
. . . . in order to “prove” that he was Afro-Brazilian, [Siqueira’s] lawyers needed to find some criteria. He went to seven dermatologists who used something called the Fitzpatrick scale that grades skin tone from one to seven, or whitest to darkest. The last doctor even had a special machine.
“Apparently on my face I’m a Type 4. Which would be like Jennifer Lopez or Dev Patel, Frida Pinto or John Stamos. On my limbs I would be Type 5, which is Halle Berry, Will Smith, Beyonce and Tiger Woods,” he said.
Like most people he has different skin tones on different parts of his body. But in none of these tests did he come out as lighter skinned.
He says the whole thing struck him as completely bizarre because identity, he says, is made up of more than just physical characteristics. [JAC: but to me, the important thing is whether discrimination is based on more than just physical characteristics.]
But this wasn’t just an isolated incident.
Mandatory for all government jobs
A few weeks ago, these race tribunals were made mandatory for all government jobs. In one state, they even issued guidelines about how to measure lip size, hair texture and nose width, something that for some has uncomfortable echoes of racist philosophies in the 19th century.
“It is something terrible. I believe this kind of strategy can weaken the support of society for affirmative action policies,” says Amílcar Pereira, an associate professor at the School of Education in the Federal University of Rio, who studies race relations. “These policies have huge support … the majority of Brazilian society supports affirmative action.”
I don’t know what to make of this. Clearly the Brazilian government is not construing race as a purely social phenomenon, since it’s based on differences that are clearly inherited (black couples have black children, and so on), and on not just skin color, but hair texture, nose width, and other traits that do co-segregate based on geographic origin.
In what sense, then, is race a “social construct” in Brazil? If race was purely a social construct with no biology behind it, then you could become benefit from affirmative action simply by declaring that you were a minority, which was what people were accusing Siqueira of. You can declare your gender, after all, so why not your race? But people don’t like the latter, as witnessed by the case of Rachel Dolezal, who declared she was black when she had no African-American genes and was of purely European descent. People wouldn’t accept that, and she was forced to resign as director the NAACP (a black organization) in Spokane, Washington.
But maybe this kind of physical measurement in Brazil isn’t so bad after all. I say this because, historically, discrimination against people was based on physical characteristics—largely skin color, but also the biological co-segregates: hair texture, nose configuration, etc. If you want to remedy discrimination based on those traits, then you find out empirically how that discrimination works, which appearances result in discrimination, and then confer advantages to those with the traits most discriminated against. That’s a purely empirical approach to the problem, and although you can call it a “social construct” approach, you’d be distorting the situation, which involves real biological differences.
In the meantime, I’m still not quite clear what people mean when they say “race” is purely a social construct. As a biologist, I can’t find any interpretation of that claim that makes much sense. But in the meantime, I think we can recognize the biology behind racial classification while still working to dismantle the bigotry that goes along with it. After all, there are medical, scientific, and evolutionary questions that rest on the genetic structure of our species.
As a biologist, I’ve learned that there are two related issues that are taboo for academics to discuss openly. The first is the issue of “races”—or genetic differences between human populations. Cultural anthropologists tell us that races are “social constructs.” Well, there’s a bit of truth in that, insofar as there is no finite number of races that can be unambiguously demarcated from each other. But there are genetic differences between groups, and clustering algorithms can divide populations into five or six fairly distinguishable groups corresponding to their geographic localities. Those differences in marker genes undoubtedly evolved via either genetic drift or natural selection in early human populations that were geographically isolated.
But the issue of whether there are genetically-based differences in behavior, physiology, mentation, and other non-physical attributes of populations is simply off the table. It’s not just that we shouldn’t investigate them (for one can make a case that such research might itself have invidious social consequences), but that those differencesdon’t exist. I’ve even heard people called “racists” by cultural anthropologists—one of the worst fields for ideologically motivated scholarship— simply for suggesting that there might be behavioral genetic differences between human groups. You can discuss the issue, but there’s only one position considered acceptable.
My own take is that the separation of human subgroups has been so recent that there hasn’t been a lot of time for extensive genetic differences to evolve, though clearly there’s been time for marked physical differences to evolve. And it’s clear that human intermixing, facilitated by transportation and increased mobility, will tend to efface all of these differences. But we shouldn’t assert that any trait beyond the most obvious physical differences between groups shows complete equality among them.
When it comes to the sexes, though, it’s a different matter. In the hominin lineage males and females have been coevolving (either cooperatively or antagonistically) for 6 million years or so—ample time for differences in behavior, wants, thought patterns, and so on to evolve, just as morphological differences between men and women have clearly evolved. Do those genetic differences in thought and behavior exist? I suspect they do, at least for traits connected to sexuality and sexual behavior. Just as animals ranging from flies to mammals show consistent (though not universal) patterns of male/female differences in sexual behavior—differences explainable by sexual selection—so I expect the human lineage evolved similar patterns. After all, males are larger and stronger than females, and you have to explain that somehow. How do you do so without explaining evolved differences in behavior—probably based on sexual selection?
Yet the idea that males and females show evolutionary/genetic differences in behavior is also anathema in liberal academia, and for the same reason that population differences are anathema. Such differences, so the thinking goes, would support either racism (on the part of populations) or sexism (on the part of males and females). But of course that thinking is false: we can accept evolved differences without turning them into social policy. And it’s of interest to many evolutionists, including me, to know the extent to which groups and sexes have evolved along divergent pathways.
Still, many feminists, liberals, sociologists, and cultural anthropologists deny any such divergence. Yes, men and women differ in body size, strength, and structure, but there are, so they say, no such differences in the brain and behavior. In all other traits, so the trope goes, men and women are equal. And given equal interests and talents, then the only thing enforcing anything other than a 50% representation of men and women in professions must be cultural pressures: viz., sexism. Thus, unequal representation in professions is prima facie evidence of sex discrimination. But as Jon Haidt mentioned in the lecture I posted the other day (watch the video; it’s good!), one first has to determine the cause of such unequal representation before one decides what to do about it.
At any rate, in the humanities and especially cultural anthropology, which in its ideological slant really counts as (sloppy) humanities rather than science, these attitudes are not only religious in nature, lacking empirical substantiation, but are also theological in enforcement. Authors (as I’ve pointed out recently) assume what they want to prove, and then go ahead and collect just those data that support their hypothesis. Confirmation bias is rife. This is what theologians do, not scientists.
The paper I’m highlighting today (link and free download below) is by Charlotta Stern, associate professor and deputy chair of the sociology department at Stockholm University. She is a brave woman, for her paper aims at calling out those sociologists who simply refuse to consider biology as an explanation of sex-distinguishing behaviors. As she says, not pulling her punches:
The present investigation is informed by my long and ongoing experience as a sociologist at Stockholm University. My teaching and research often touch on gender issues. I have served on about five thesis committees that addressed gender sociology or related matters, and I have participated in dozens of seminars that touch on gender sociology. My relationships with my colleagues and students are not heated. When I raise ideas that would challenge the sacred beliefs, I do so only at the edges. I have seen how people react when I or another suggests that maybe there is a difference in math skills between men and women, or that men and women have different preferences and motivations. In my experience, gender sociologists frown upon such remarks about innate differences in aptitude or motivations. I perceive deep and widespread taboo and insularity among gender sociologists. It saddens me. I feel impelled to make available some expression of my concern, hoping that students and others will hear it before sinking into the sacred beliefs and sacred causes addressed here.
Her method was simple, and somewhat subjective. She examined a set of 23 highly-cited articles in sociology journals, all of which cite a classic paper in the field, “Doing gender,” by Candace West and Don Zimmerman (1987); reference and free link below).
West and Zimmer concluded (or decided in advance) that behavioral and non-physical differences between men and women were “constructed” based on their genitalia, so that all differences we observe in later life are the result of socialization. As Stern notes,
“Doing gender” is presented as part of a lamentable system of social control. The paper’s final para- graph reads:
“Gender is a powerful ideological device, which produces, reproduces, and legitimates the choices and limits that are predicated on sex category. An understanding of how gender is produced in social situations will afford clarification of the interactional scaffolding of social structure and the social control processes that sustain it. (West and Zimmerman 1987, 147).”
Stern examined 23 highly-cited sociology papers published between 2004 and 2014 (two per year) that themselves cited West and Zimmer’s influential paper. Then, developing a spreadsheet, she coded each of the articles as whether or not they took the hypothesis of biological differences between men and women as a serious possibility. Her classification was as follows:
Neutral. Discussions of gender differnces but no discussion of their biological bases, nor dismissal of them. (4 articles).
Blinkered. These are the articles in which, according to Stern, biological differences are relevant hypotheses, but are either ignored or dismissed out of hand (15 articles).
Unblinkered. Stern found only one article that considered biology as a possible explanation for sex differences in things like time spent with children, savings for education, and other “family processes.” Stern says the article has a “nuanced discussion of causality.”
Not rated. These articles “do not deal with matters for which biological difference ideas would clearly be relevant.” Four articles.
Here is Stern’s list of the articles and their ratings:
Now of course you can debate Stern’s methods and assessments, but what’s clear even without this analysis is that it’s taboo in much of academia to suggest that measurable differences between populations or sexes (excluding the most obvious physical differences) have any biological basis. But there should be no taboos in academics. One can debate the wisdom of investigating some questions (e.g., “Are Jews genetically acquisitive?”), but what one should not do is assume what’s true before investigating it. And, as Jon Haidt noted, if you don’t know the empirical basis for differences that are considered problematic (such as the underrepresentation of women in mathematics), you’re hampered from addressing them.
Stern’s conclusion is low key (my emphasis):
One cannot draw quantitative estimates on the basis of my investigation, but its findings are consistent with an image of gender sociology as a subfield that has insulated its sacred beliefs from important scientific challenges.
I have extensive first-hand experience with gender sociology’s insularity. But I also know of pervasive preference falsification (Kuran 1995), and I have seen students awaken with an ‘a-ha!’ moment when exposed to unorthodox thinkers such as Catherine Hakim (1995; 2000; 2008). I believe reform is possible. Whether people should ‘do gender’ less, and how they should ‘do gender,’ are questions worthy of personal reflection, scholarly exploration, and public discourse. More definite, to my mind, is that people should do less insularity.
As I’ve often said, the question I’m always asked after my public lectures on evolution is this: “Are humans still evolving?” And my answer is always the same: “Yes, but the evidence we have for evolution occurring right now involves traits that aren’t that interesting.” When people ask that question, what they really want to know is whether humans are getting better looking, more athletic, smarter—whether we’re turning into a race of superheroes. And all I can tell them is that, over the last 10,000 years, our species has evolved in some places to be more lactose-tolerant, in other places to be more resistant to malaria, and in still other places to adapt to the low-oxygen conditions of living at high altitude. But that’s still in the past (see a summary here).
As for evolution in the present day, we have “real time”, “horizontal” observations for things like selection in women for earlier age of first birth, later age of last birth and (also in women) increases in height in some places and decreases in others. Studies in the U.S., which haven’t been conducted elsewhere, show a recent evolution of reduced cholesterol levels and lower blood pressure, and an increased age of menopause (see here and here). But those results don’t excite people much. In general, though, as long as there is variation in some genes that causes variation in reproductive success, humans will continue to evolve. That hard part is documenting which genes and which traits are associated with reproductive success, and that means laborious studies correlating people’s genes and traits with their reproductive output.
One such study, by Jonathan Beauchamp, was just published in the Proceedings of the National Academy of Sciences (free download, link and title below). It used genetic, phenotypic (“trait”), and reproductive-success data from about 12,000 U.S. males and females of European ancestry, born between 1931 and 1953, all examined in a Health and Retirement Study (HRS). Using people of that age ensured that most of them had completed their reproduction, so the number of children they had (child mortality wasn’t counted) served as an index of their reproductive success (RS). And, of course, traits conferring a greater RS are those that selection will favor in populations. The currency of natural selection is reproductive output!
The authors did two types of analyses of the data.
a. Trait analysis. The authors used data on body mass index (BMI), educational attainment (EA), fasting glucose concentration (GLU), height (HGT), schizophrenia (SCZ), plasma concentration of total cholesterol (TC), and age at menarche (AAM), the last trait studied, of course, only in women. They then correlated the values of each trait with the reproductive success of their bearers:
The results? As the table below shows (asterisks denote statistical significance at the p < 0.01 level), there was evidence of phenotypic “selection” for stouter males and females, selection again educational attainment in both males and females, and selection for shorter women (but not men).
Taken at face value, these data show that Americans in that year class are experiencing selection to get chubbier, to stay in school fewer years, and, in women, to get shorter. But of course there are problems, because they are looking at a correlation between a trait and the number of children produced by people with different trait values—and there’s no genetics here. Perhaps there are cultural or other reasons, for instance, for the correlation between staying in school for less time and having more children. One example would be if people leave school to have children, or put off having children while they’re in school. You’d get the appearance of selection on the trait, but there might not be any genes involved, so there would be no evolution. Because of these issues, the author did a study explicitly incorporating genes.
b. Genetic analysis. This involved looking at the DNA of every person at many sites (between 80,000 and 400,000 DNA positions, depending on the trait), and finding those combinations of gene positions best correlated with the values of the trait (I’m simplifying matters here, but it’s not important). These combinations of DNA positions were then considered to be the genetic sites that could be influenced by selection on that trait. Having done that, Beauchamp could then see if those combinations of genes were themselves correlated with reproductive success, and thus could be under selection. After having done the appropriate statistical corrections for multiple tests, Beauchamp found the following results:
So the negative correlation between genes associated with staying in school and the reproductive success of their carriers was still observed. In other words, both men and women were undergoing natural selection for fewer years of schooling. There was still a marginally significant (p < 0.1) association between age of menarche and reproductive success, in line with previous studies showing that American women are evolving to reach menopause later. There was no significant association between genetic constitution and reproductive output on the other traits.
Three questions remain:
How strong is selection against educational attainment? Answer: not very strong. The estimate given by Beauchamp is that we’re staying in school about one week to six weeks less per generation. If you take 25 years per generation, it would take about 35 generations, or 875 years, for selection to reduce our education by about a year.
So are we getting dumber? Answer: nope, because phenotypically we’re staying in school longer. That is, the cultural trends on Americans show that between 1876 and 1951 (75 years), Americans’ EA increased by 6.2 years—an increase of about two years of education per generation. That far outstrips the genetic change, so, even if the selection data are correct, we’re still going to keep increasing our educational attainment. What we’re seeing is the reproductive advantage of leaving school a bit earlier is being overcome by the cultural trend to stay in school longer.
What other problems are there? Beauchamp does a good job in pointing out the caveats of his study. I won’t recount them in detail, but they include the possibility that having more children doesn’t mean that you’re going to have better children—that is, that those extra kids may not themselves reproduce as well as kids from smaller families. That could weaken or even reverse the direction of selection. And, of course, this correlation was seen in only people from one or two generations, and there’s no guarantee that it will continue over the long term—or even that we’d see the same kind of selection in other countries.
In the end, we have a suggestive result, but one that needs a lot more work before it’s accepted as definitive. That work would somehow have to look at the reproductive output of the children themselves sired by the measured individuals, and nobody is going to do that. Further, the genetic result is being overcome by cultural trends, so we don’t really have to worry that we’re breeding a generation of dropouts!
_________
Beauchamp, J. P. 2016. Genetic evidence for natural selection in humans in the contemporary United States. Proceedings of the National Academy of Sciences 113:7774-7779.
Reader Dorsa Amir called my attention to a story by Susan Dominus in today’s New York Times magazine, “The mixed-up brothers of Bogotá.” It tells a bizarre tale of swapped twins that gives clues about the genetic basis of human behavior.
It turns out that two pair of identical twins were born in Colombia at the same time, and one individual of each pair was accidentally swapped. Each pair was then raised as assumed fraternal twins. Only much later was the mistake noticed, discovered by a woman who knew one twin and encountered the other in working a butcher shop, assuming it was her friend who was playing a joke when he didn’t recognize her. After a period of trepidation, the real identical twins met each other and discovered that their similarity ran far deeper than appearance.
Here they are: each twin was raised with an individual from the other pair:
(From left) Jorge Enrique Bernal Castro, William Cañas Velasco, Carlos Alberto Bernal Castro and Wilber Cañas Velasco. Credit Stefan Ruiz for The New York Times
Identical twins separated at birth are precious resources for those interested in the genetic basis of human behavior and morphology, for their similarity gives us a clue to how much of human variation has a basis in genes versus environments—or the interaction between the two factors. We know from the photo above that morphology (in this case, both shape and facial features) are largely determined by genes: although the first and third twins from the left, as well as the second and fourth, were raised together, the pairs that look the same are #1 and #2, as well as #3 and #4. Despite the identical twins having experienced different environments from the moment of birth, their much closer resemblance compared to the assumed fraternal twins (actually unrelated individuals) is clear . This, and other similar work with twins, shows us that the variation in appearance among humans is largely based on the variation in their genes rather than in their upbringing.
But we already knew that: offspring tend to resemble parents even when the offspring are separated when young from their parents, and reared in a different environment. But identical twins raised apart are one of the best tests of the genes-versus-environment—nature versus nurture—hypothesis (yes, the genes do interact with environment) for behavior as well. If identical twins raised apart, which share all of their genes, are more similar to each other for a given trait than are real fraternal twins reared together (which, like regular siblings, share half of their genes), that suggests that the genetic contribution to the trait’s variation is more important than the environmental contribution. For if similarities in behavior were caused entirely by the environment, then identical twins reared apart should be less similar in behavior than fraternal twins reared together: the latter were raised in a common environment (more or less), while the former weren’t.
In general, as the article notes, earlier studies have shown a remarkable influence of genes on human behavior: identical twins raised apart are often eerily similar:
Arguably the most intriguing branch of twins research involves a small and unusual class of research subjects: identical twins who were reared apart. Thomas Bouchard Jr., a psychologist at the University of Minnesota, began studying them in 1979, when he first learned of Jim and Jim, two Ohio men reunited that year at age 39. They not only looked remarkably similar, but had also vacationed on the same Florida beach, married women with the same first name, divorced those women and married second wives who also shared the same name, smoked the same brand of cigarette and built miniature furniture for fun. Similar in personality as well as in vocal intonation, they seemed to have been wholly formed from conception, impervious to the effects of parenting, siblings or geography. Bouchard went on to research more than 80 identical-twin pairs reared apart, comparing them with identical twins reared together, fraternal twins reared together and fraternal twins reared apart. He found that in almost every instance, the identical twins, whether reared together or reared apart, were more similar to each other than their fraternal counterparts were for traits like personality and, more controversial, intelligence. One unexpected finding in his research suggested that the effect of a pair’s shared environment — say, their parents — had little bearing on personality. Genes and unique experiences — a semester abroad, an important friend — were more influential.
But, as Dominus notes, these studies aren’t perfect for several reasons: identical twins could be adopted into similar environments by the agencies; they might have known each other, so there’s the possibility of some shared environmental influences; there could be a form of self-selection, so that identical twins who share quirky behaviors are more likely to be found since the media reports on them more often or they’re more likely to be discovered. In the case of the Castro and Velasco “brothers,” most of these problems don’t exist: there was no adoption and no self-selection.
The separated twins were studied by researchers, and I’ll let you read the article to see the results. Let me just say that in general they align with the earlier studies of twins separated at birth: a strong genetic component to behavioral variation—and the behaviors involved are sometimes quirky. Here’s a snippet describing how the two pair of twins behaved after they hung out together before they were formally tested:
The four young men all knew one another well by then. Over the past six months, they had gone on outings and shared meals, talked about women, family, money, values. Even weeks in, each had stared, still unnerved and amazed, into the eyes of his identical brother. They had measured, assessed and inspected. They stood back to back, comparing height (those raised in the city were taller than those from the country [JAC: this shows that there is a big environmental influence on height, as we’ve known for some time that nutrition is important here]); Carlos had crushed Wilber in a food-eating contest, William had vanquished them all when they arm-wrestled. In the stands at a soccer match, Carlos watched, in fascination, as William’s hand reached down his jeans to scratch his backside: Jorge did the same thing, Carlos told Wilber. Over dinner one night, Jorge noted that Carlos and Wilber both leaned in at the same odd angle toward their plates.
But there were differences between the identical twins, as well: researchers showed that in some respects they were less similar than expected (though it’s not reported whether they were still more similar than were the unrelated twins who were reared together). And epigenetics is part of the story, too.
This is a fascinating tale of both human and scientific interest, and although it’s a bit light on the science for my taste, it still shows the kind of natural experiment we need to determine what proportion of human variation is based on genetic differences, what proportion on environmental differences, and what proportion on the interaction of genes and environment. Ethics dictates that we can’t do the kind of experiments on humans that we can on flies and cows: separating individuals at birth and seeing how much difference in behavior and appearance can be created by rearing siblings (identical or fraternal) in different environments. Data so far show that a surprisingly large amount of variation in human behavior rests on variation in genes, but (as I noted) these studies aren’t perfect. Still, they should give pause to those who believe (often based on political ideology) that genes don’t play much of a role in the diversity of behavior among individuals in human population.
JAC: We met Dorsa Amir a while back when she sent us photographs of the Barbary macaques she worked on in Morocco’s Atlas Mountains (she also sent a photo of her orange tomcat Emerson). She recently called my attention to Amy Cuddy’s popular TED video on “power posing”, as Cuddy’s conclusion about hormones and behavior is relevant to Dorsa’s thesis work. Not only that, but Cuddy’s talk, now up to 25,758,838 views, is the second most popular TED talk of all time. But, referring me to an analysis on another site, Dorsa told me that Cuddy’s conclusions were dubious—or at least non-repeatable. I asked her to write a post about it, which is below. I’ve also put in Cuddy’s talk, which I recommend you watch, so you can see her claims and how much the audience loved it.
First, Dorsa’s information:
I’m a PhD student in biological anthropology at Yale University. My research interests include the physiology and psychology of contemporary humans, with a focus on small-scale societies. I spend my summers in South America with the Shuar of eastern Ecuador, as part of the Shuar Health and Life History Project
Personal site: www.dorsaamir.com
*******
Pulling the Plug on Power Posing
by Dorsa Amir
If you’ve seen just one TED talk, I would bet it’s called “Your body language shapes who you are” by Amy Cuddy, now an Associate Professor at Harvard Business School. This is a pretty safe bet, for in the three years since its release, Cuddy’s talk has racked up more than 25 million views, making it one of the most popular TED talks of all time.
In the talk, Cuddy presents data from her 2010 article in Psych Science which makes the following claim: by simply changing your posture to a “high-power” pose (i.e., taking up more space and opening your limbs), you can instantly trick your body into thinking it’s more powerful. The authors tested this claim by having 42 participants give saliva samples, engage in either a high-power or a low-power pose for two minutes (depicted below), then give another saliva sample.
The saliva tubes were then sent off to a lab and analyzed for two specific hormones: testosterone and cortisol. Interestingly, the power posing appeared to have a significant effect on hormone levels: high-power poses were associated with a rise in testosterone and a drop in cortisol, and low-power poses with the opposite. So not only did the posing make you feel more powerful, it also made your body more powerful by fiddling with your hormone levels and making you literally embody that power.
DataColada vs. Power Posing:
So why are we talking about a video from three years ago? Well, in a blog post on Friday, the talented folks over at DataColada tackled the evidence supporting power posing. As it turns out, as shown in a recent paper in Psych Science, the power posing effect doesn’t seem to replicate. I suggest checking out the full blog post for details, but here are their main points:
The replication method in the new study is precise enough to be informative.
The original sample size of N=21 per cell had less than 6% statistical power to detect the effect, even if it existed. This is problematic: even if you do find the effect, being so extremely underpowered decreases the likelihood that the effect is real. In other words, the smaller your sample size, the greater the likelihood of a false positive.
As the bloggers claim, “if studies only get published when they show an effect, the fact that all the published evidence shows an effect is not diagnostic”. So what you can do, and what they did, is run a fancy statistical method called a p-curve analysis; a method that lets you rule out selective reporting as the only explanation for a set of significant findings.
Check out the two example graphs below, comparing a good to a bad p-curve. On the X-axis is the p-value reported in any one study, and on the Y-axis is the percentage of studies that have that p-value. This lets you compare p-values for the same experimental protocol across many studies and see what the curve looks like. You want a right-skewed curve because that indicates that proportionally more studies are reporting effects that are significant at lower values. So, if power posing is real and effective and robust, as assessed in 33 separate studies, you should see a right-skewed p-curve. You don’t want to see a flat curve, because that means that all the different p-values are equally likely to occur.
Here’s what the actual p-curve analysis looks like:
We see not the right-skewing that would convince us that this is a real effect, but rather a flattish curve that should make us question the finding.
The DataColada folks conclude, “At this point the evidence for the basic effect seems too fragile…to advocate for people to engage in power posing to better their lives.”
Hormones & Behavior:
I’d like to take a minute here to add my two cents. In our lab at Yale, my colleagues and I spend a lot of time thinking about hormones—our lab meetings are affectionately referred to as “Hormone Happy Hour”—and we spend a lot of these meetings discussing the relationship between hormones and behavior. As it turns out, hormones, like everything else in biology, are kind of complicated. While testosterone and cortisol have been implicated in basically every human behavior, let me share the basics. Testosterone plays a big role in male development by turning baby boys into pubescent boys, and in adulthood it makes muscles and semen happen. There’s no clear evidence that testosterone contributes to variation in aggression between individuals, but it’s likely to contribute to differences in aggression between the sexes [1], and there’s really good evidence that it helps you build big muscles[2,3]. Cortisol, contrary to popular belief, doesn’t cause stress, it’s just a response to stress: when your body senses that it’s in need, cortisol increases the amount of glucose in your bloodstream so your cells can have greater access to energy.
In general, hormones like testosterone and cortisol are dynamic. Both hormones have a diurnalrhythm, which means they change throughout the day. They’re also influenced by dozens of variables: the obvious ones like age, sex, and weight help determine clinical guidelines for what “normal” levels look like. One big problem here, though, is that if you look at men from different populations, such as hunter-gatherers, their absolute levels of both testosterone[4] and cortisol[5] are sometimes almost half of those found in American men.
Why is all this relevant to power posing? Well, there are three main points to be made: (1) hormones have complicated effects on behavior, (2) hormones often change throughout the day, and (3) the density and affinity of hormone receptors are potentially just as important as absolute hormone levels (excellent evidence here). How did Cuddy and colleagues control for these phenomena? In short: they didn’t. They took two saliva samples, prior to and following the manipulation, and attempted to control for diurnal rhythm by scheduling testing in the afternoon – a massive six hour window that stretched from 12:00pm-6:00pm. Due to the dynamic nature of cortisol, the standard cortisol assessment protocol generally involves something like two samples a day for 2-3 days. This means cross-sectional samples, like the one in the power posing study, must be analyzed with a grain of salt.
The best way to address the question of whether or not changes in hormone levels caused by the treatment are in turn causing a change in behavior is to run a mediation analysis. This is a method in which you try to test causality among three variables; in this case, you’d want to see that changes in hormones are mediating, meaning causally linking, high-power poses to more confidence. You don’t necessarily need to know all the details of this method (more info here), but the basic point is that the authors, for some reason, do not include this simple test in their paper. Without knowing what people’s hormonal profiles look like (by taking several samples across many days), it’s really hard to say whether or not you’re measuring a “trait” difference or a “state” difference.
A long-running debate in my field is the relationship between statistical significance and biological significance. Even if this treatment did cause significant changes in hormone levels (which the DataColada folks suggest we should doubt), we’re still left with open questions: does this treatment have any biological significance and if so, what are the proximate mechanisms? Even if we see differences in confidence level post-treatment, can we really say it’s because of hormonal changes?
