I was recently reading Sexual Behavior in the United States: Results from a National Probability Sample of Men and Women Ages 14–94. At N ~ 6,000 it’s a large sample of American sexual behavior around 2010. There was one descriptive result which I thought was interesting, though not surprising. Before the age of 25 it seems that women are more likely to have sex in a given year than an equivalent age man. After the age of 25 this starts to reverse, and men are more likely to be having sexual intercourse in a given year. The dynamics underlying this phenomenon seem to be easily subject to various speculations, so I’ll leave that to readers. Rather, I offer the graph (data drawn from the paper linked above):
My post below elicited this response:
Here are a couple of cases which seem to defy easy classification.
A “chimera”. This is a person who has cells derived from two zygotes. It can happen if two fertilized eggs merge very early in development. The individual may appear entirely normal (there may be chimeras reading this who are unaware of their condition); but the cells in their body will come from two quite distinct origins. If the original zygotes were male and female, then the adult individual will have some cells in their body with the XY (male) chromosomes, and others with the XX (female) chromosomes. There may be no external sexual ambiguity as long as the sex organs all come from the one lineage; in general all kinds of sexual ambiguity might arise.
Second case; more common (though still unusual) is where an individual is genetically of one gender, and phenotypically of the other. This can be either an XX individual who develops with external male genitalia; or XY who develops with female genitalia. This is usually caused (I think) by excess or deficit of the appropriate hormones during fetal development.
For most people, gender is unambiguous. But there is no sharp dividing line or easy way of classifying that covers all individuals.
The examples I’m considering are entirely independent of psychology or choice. They are real physical conditions in which the conventional physical basis for determining gender becomes ambiguous.
The New York Times has an article up on a new I.O.C. ruling on who can compete as a woman. Basically they look at testosterone levels. This seems a different tack than cases where women were banned from competing as women because they had a male karytoptype (AIS). This article came on my radar because I had already read this op-ed from about a week ago, You Say You’re a Woman? That Should Be Enough. This sentence jumped out at me:
Second, when it comes to sex, sports authorities should acknowledge that while science can offer evidence, it cannot dictate what evidence we should use. Scientifically, there is no clear or objective way to draw a bright line between male and female.
What do people think of this assertion? I’m aware of intersex individuals. But if we start to assert that dioecy is just a “social construct” then let’s revisit species concepts. I’m sure there are some farmers and loggers who might assert that one can’t draw objective bright lines between populations. Distinctions between male and female in most species is much more clear and distinct than various taxonomic categories.
I’ve been commenting on internet porn for nearly 10 years. One reason is that as someone who graduated high school in the spring of 1995 I’m probably in the very last cohort of American males for whom pornography was an item subject to scarcity. Those who are 2-3 years younger already experienced a totally different world. The furtive quest to find a friend of a friend whose dad was less than vigilant in guarding his porn stash was a rite of adolescent male passage in my cohort, but would seem totally laughable by 1997. There’s a lot of commentary on the effect of porn on society and sexual relations, but from what I can tell nothing much has really changed between then and now, except that hardcore porn has become harder. Before I see hard data I’m skeptical that American males accept more perversion because of watching porn. Read the Kinsey Reports; farm boys long knew some farm boys lost their virginity to animals.
All this must be kept in mind when reading pieces tinged with moral panic, such as this one in The New York Times, So How Do We Talk About This?, which details the reaction of parents to their children discovering porn. There are few specific elements which strike me as manifestly stupid. For example:
Bonnie, a university administrator in North Carolina with a teenage son and two stepdaughters, realized only after discussing the matter that she and her husband had been sending unintended messages by emphasizing safety and self-protection with the girls and limits with her son.
“Later, we realized how terribly, albeit unconsciously, sexist that was,” she said.
The magazine Foreign Policy recently had a “sex” issue out. This issue is particularly famous for Mona Eltahaway’s jeremiad against Arab male culture, and their attitudes toward women. Over at bloggingheads.tv Charli Carpenter expresses some concern that the issue seemed so singularly focused on Arabs, as if women’s rights is a problem with particular salience for Arab Muslims. As it is, she admits that as a matter of truth it may be so, but still has qualms about essentialization.
Now, I like to think in terms of distributions, and don’t find essentialization particularly useful on a fundamental level. But, my personal observation is that the term ‘essentialization’ tends to be used when there are phenomena brought to light which make people uncomfortable. For example, I rarely hear essentialization being nearly a great a problem when talking about Republicans or Western Christian conservatives.
But it does make to wonder: how bad are Arab countries when it comes to women’s rights? Let’s look at the World Values Survey. There are two questions in the survey which have a lot of normative baggage:
- If jobs are scarce: men should have more right to a job than women
- It is an essential characteristic of democracy that women have the same rights as men
As a matter of pedantic accuracy obviously it is not an essential characteristic of democracy that women and men have the same rights. Ancient Athens and America before the 1920s are generally considered democracies. But, the question is a rough gauge of attitudes toward male and female legal equality. I relabeled these questions as “economic” and “legal” equality. They are given as percentages, so I converted the categories into numbers, and then took the weighted average. These two indices measure the two dimensions of equality, with higher values favoring more equality. Below I generated a scatterplot showing the relationship between the two (naturally positively correlated). I’ve also attached table data after the figure.
It’s a fun fact that there are an order of magnitude more bacterial cells in your body than your own cells. Not only that, it’s well known that we wouldn’t flourish, let alone survive, without our gut “flora,” which digest material which would otherwise pass through out system. Not only are microbes good for us, but they’re also bad for us. The evolutionary flexibility of microbial pathogens is one of the major arguments for why sex exists among multiceullar species: it allows them to adapt to rapidly fluctuating disease pressures. Therefore, obviously the ecology of multicellular organisms’ microbial flora is essential to properly characterize. One element of the project involves genomics. This is not so easy for microbes because we don’t have the reference sequences of most of these organisms. We rely mostly on species which are easy to culture, and that does not include most lineages in the wild. That being said, there are workarounds, such as looking at the 16S rNA sequence, which is strongly constrained in bacterial lineages (i.e., it can serve as a “clock” to measure divergence of very deeply separated lineages).
With that, a new paper, Promiscuity in mice is associated with increased vaginal bacterial diversity:
I just read a very strange article in the journal Evolution, Sex reduces genetic variation. In it the authors argue that contrary to conventional wisdom and evolutionary orthodoxy the rationale for the prevalence of sex amongst eukaryotic organisms is not maintenance of genetic variation, but rather a constraint upon genetic variation! This is a very peculiar view, and as someone not immersed in the literature on sex totally surprising to me.
The standard model is simple: sex allows organisms to swap genetic material and generate new combinations. This is at a particular premium for large, complex, and slow-breeding lineages, as is the norm amongst eukaryotes. In contrast, bacteria and their ilk have huge population sizes to draw from, and are quite literally protean in their ability to shift strategies to climb whatever adaptive landscape nature throws at them. Carl has a nice review of a paper in Science which reported just this finding in keeping with expectation. Increase the pathogen pressure, and eukaryotes which exchange genes marginalize those which do not because they can dodge the punches that their evolutionary adversaries throw in their direction.
A quintessentially sexy topic in biology is the origin of sex. Not only are biologists interested in it, but so is the public. Of Matt Ridley’s older books it is predictable that The Red Queen has the highest rank on Amazon. We humans have a fixation on sex, both in our public norms and our private actions. Why?
Because without a fixation on sex we would not be here. Celibates do not inherit the earth biologically. This answer emerges naturally from a Darwinian framework. And yet more deeply still: why sex for reproduction? Here I allude to the famous two-fold cost of sex. In dioecious species you have males and females, and males do not directly produce offspring. The increase of the population is constrained by the number of females in such lineages (male gametes are cheap). There is no such limitation in asexual lineages, where every individual can contribute to reproductive “primary production.” Additionally, the mating dance is another cost of sex. Individuals expend time and energy seeking out mates, and may have to compete and display for the attention of all. Why bother?
The answer on the broadest-scale seems to be variation. Variation in selective pressures, and variation in genes. Sex famously results in the shuffling of genetic permutations through recombination and segregation. In a world of protean change where one’s genes are critical to giving one the edge of fitness this constant flux of combinations results in more long term robusticity. What clones gain in proximate perfection, they lose when judged by the vicissitudes of the pressures of adaptation. In the present they flourish, but in the future they perish. Sex is the tortoise, clonal reproduction is the hare.
And yet science is more than just coarse generalities; biology especially so. The details of how sex emerges ad persists still remains to be fleshed out. The second volume of W. D. Hamilton’s collected papers, Narrow Roads of Gene Land, is the largest. Mostly because it was not edited appropriately (he died before it could be). But also perhaps because it is the volume most fixated upon the origin and persistence of sex, which is a broad and expansive topic.
A new paper in Nature tackles sex through experimental evolution. In may ways the answer it offers to the question of sex is old-fashioned and straightforward. Higher rates of sex evolve in spatially heterogeneous environments:
Eric Michael Johnson has a fascinating piece in Psychology Today, Sex, Evolution, and the Case of the Missing Polygamists. I want to spotlight a few paragraphs:
Keep in mind that in terms of interpreting such genetic evidence we are of necessity confined to a fairly recent time depth (and remember, by “recent” someone like me means the last 10,000 years or so). For this time period multiple lines of evidence do indeed suggest that humans were moderately to extremely polygynous and that women were moving between groups more than men were.
However, humans have been around for far longer than 10,000 years, with conservative estimates placing the emergence of modern Homo sapiens at about 200,000 years ago. A genetic record extending back 10,000 years is remarkable, but it’s essentially adding only three more novels to our existing timeline. There is also something very important to consider that dramatically influenced human behavior within the last 10,000 years: the invention of agriculture. Prior to about 12,000 years ago all humans were hunter-gatherers and lived a migratory existence. With the advent of farming some human societies began to remain sedentary for the first time in our history. This change had serious impacts on human life and behavior. Just as Alzheimer’s dramatically altered the content of Agatha Christie’s work, so agriculture radically transformed human society and, by consequence, sexual behavior.
Mutations are as you know a double-edged sword. On the one hand mutations are the stuff of evolution; neutral changes on the molecular or phenotypic level are the result of from mutations, as are changes which enhance fitness and so are driven to fixation by positive selection. On the other hand mutations also tend to cause problems. In fact, mutations which are deleterious far outnumber those which are positive. It is much easier to break complex systems which are near a fitness optimum than it is to improve upon them through random chance. In fact a Fisherian geometric analogy of the affect of genes on fitness implies that once a genetic configuration nears an optimum mutations of larger effect have a tendency to decrease fitness. Sometimes environments and selection pressures change radically, and large effect mutations may become needful. But despite their short term necessity these mutations still cause major problems because they disrupt many phenotypes due to pleiotropy.
But much of the playing out of evolutionary dynamics is not so dramatic. Instead of very costly mutations for good or ill, most mutations may be of only minimal negative effect, especially if they are masked because of recessive expression patterns. That is, only when two copies of the mutation are present does all hell break loose. And yet even mutations which exhibit recessive expression tend to generate some drag on the fitness of heterozygotes. And if you sum small values together you can obtain a larger value. This gentle rain of small negative effect mutations can be balanced by natural selection, which weeds does not smile upon less fit individuals who have a higher mutational load. Presumably those with “good genes,” fewer deleterious mutations, will have more offspring than those with “bad genes.” Because mutations accrue from one generation to the next, and, there is sampling variance of deleterious alleles, a certain set of offspring will always be gifted with fewer deleterious mutations than their siblings. This is a genetics of chance. And so the mutation-selection balance is maintained over time, the latter rising to the fore if the former comes to greater prominence.
The above has been a set of logic inferences from premises. Evolution is about the logic of life’s process, but as a natural science its beauty is that it is testable through empirical means. A short report in Science explores mutational load and fitness, and connects it with the ever popular topic of sexual selection, Additive Genetic Breeding Values Correlate with the Load of Partially Deleterious Mutations:
Below I note that sex matters when it comes to evolution, specifically in the case of how sexual reproduction forces the bits of the genome to be passed back and forth across sexes. In fact, the origin of sex is arguably the most important evolutionary question after the origin of species, and it remains one of the most active areas of research in evolutionary genetics. More specifically the existence of males, who do not bear offspring themselves but seem to be transient gene carriers is a major conundrum. But that’s not the main issue in this post. Let’s take the existence of males as a given. How do sex differences play out in evolutionary terms shaping other phenotypes? Consider Bateman’s principle:
Bateman’s principle is the theory that females almost always invest more energy into producing offspring than males, and therefore in most species females are a limiting resource over which the other sex will compete.
Female ova are energetically more expensive, and scarcer, than male sperm. Additionally, in mammals and other live-bearing species the female invests more time and energy after the point of fertilization but before the young exhibit any modicum of organismic independence (the seahorse being the exception). And, often the female is the “primary caregiver” in the case of species where the offspring require more care after birth. The logic of Bateman’s principle is so obvious when its premises are stated that it easily leads to a proliferation of numerous inferences, and many data are “explained” by its operation (in Mother Nature: Maternal Instincts and How They Shape the Human Species the biological anthroplogist Sarah Hrdy moots the complaint that the principle is applied rather too generously in the context of an important operationally monogamous primate, humans).
But the general behavioral point is rooted in realities of anatomy and life-history; in many dioecious species males and females exhibit a great deal of biological and behavioral dimorphism. But the direction and nature of dimorphism varies. Male gorillas and elephant seals are far larger than females of their kind, but among raptors females are larger. If evolution operated like Newtonian mechanics I assume we wouldn’t be theorizing about why species or sex existed at all, we’d all long ago have evolved toward perfectly adapted spherical cows floating in our own effluvium, a species which is a biosphere.
Going beyond what is skin deep, in humans it is often stated that males are less immunologically robust than females. Some argue that this is due to higher testosterone levels, which produce a weakened immune system. Amtoz Zahavi might argue that this is an illustration of the ‘handicap principle’. Only very robust males who are genetically superior can ‘afford’ the weakened immune system which high testosterone produces, in addition to the various secondary sexual characteristics beloved of film goers. Others would naturally suggest that male behavior is to blame. For example, perhaps males forage or wander about more, all the better to catch bugs, and they pay less attention to cleanliness.
But could there be a deeper evolutionary dynamic rooted in the differential behaviors implied from Bateman’s principle? A new paper in The Proceedings of the Royal Society explores this question with a mathematical model, The evolution of sex-specific immune defences:
Earlier I pointed to the possibility of biophysical constraints and parameters in terms of inheritance shaping the local trajectory of evolution. Today Olivia Judson has a nice post [link fixed] on how the existence of two sexes in many species results in a strange metastable tug-of-war in terms of phenotypic evolution:
In sum, the traits that make a “good” male are often different from those that make a “good” female. (Note: I’m only talking about “good” in evolutionary terms. That means a trait that improves your chance of having surviving offspring.) Since many of these traits have a genetic underpinning, male and female genes are thus being sculpted by different forces.
But — and this is the source of the tension I mentioned — males and females are formed from the same underlying set of genes. After all, in humans, whether you’re a boy or a girl comes down to whether you have a Y chromosome or not: boys do, girls don’t. The rest of the genes occur in both sexes.