Category Archives: Human sexuality

Catching Fire. The other one.

Catching Fire is apparently a very popular book and/or movie that everyone is very excited about. But Catching Fire: How Cooking Made Us Human is a different a book about some interesting research I was involved in about the origin of our genus, Homo.

You can pick up a copy of our paper on this page. We call it “The Cooking Hypothesis.” The basic idea can be summarized with these points:

1) Cooking food transformed human ecology. Many potential foods in the environment can’t be consumed by humans (or apes in general) without cooking. But adding cooking to our species-specific technology, we can access those foods effectively transforming our ecology to a much greater extent than the vast majority of evolutionary transitions, especially single-event transitions, have ever done. The total number of calories in the natural environment that become available to an ape that can cook goes up by orders of magnitude.

2) This increase in available calories left a biological signal that is very impressive. Two major changes happened in the hominid body (in early Homo erecuts/ergaster). One is an approximate doubling in body size from an earlier Australopithecine or “Early Homo” ancestor. The other is a reduction in tooth size. Less eating equipment with a body demanding so much more in energy to grow and maintain signals a fundamental change in the food supply. There may be more than one way this could have happened, but so far adding cooking to our technology seems to be the best explanation.

3) Related, this is when we see brain size, relative to body size and in absolute terms, increase. Neural tissue is picky, expensive, and costly. Having a significant increase in brain size may be related to the demands (on the brain) of adding cooking to our behavior in that the size increase is allowed by the extra energy. And, it may be related in that the larger brain may provide the capacity to have this behavior.

4) The actual act of cooking, as a technology, may or may not demand a larger brain. But the process of cooking almost certainly involves central place foraging (bringing all the food back to one place, much of the time, to cook it) and delayed consumption (as opposed to eating the food where you find it). The basic pattern for a chimpanzee-like ancestor is to eat the food where you find it. Bringing food into close proximity to other members of your group virtually guarantees direct competition for food, which makes getting to food to begin with a highly questionable thing to do. In order for cooking to work, the social interactions typical of an ape have to be modified significantly. Cooking demanded, facilitated, and made major changes in social structure “worth it” from the point of view of natural selection.

5) These changes in social structure are probably indicated as well by changes in stone tool technology. Early cookers also were early hand-ax makers, for example. Human ancestors went from making primarily expedient, one time use, very simple stone tools to making tools that required a great deal of investment in time and energy to learn the technology, get good at it, and even for the production of individual tools (including acquisition of better than average raw materials in many cases). Once the tools were made they seem to have been used, often, for long periods of time. It is hard to imagine a chimp-like creature carrying around a tool into which she invested time and energy without it being taken away. This is an important transformation.

6) Less visible but very likely is a change in social system which could be called the rise of proto marriage. Sexual arrangements of a human-like kind are very different than for chimp. The ability to allow others to possess food or invest in more sophisticated technologies may be parallel to the ability to have more or less exclusive sexual contracts among individuals. This is indicated independently in the fossil record by a large decrease in sexual dimorphism in body size. In polygynous species like chimps males are often much larger than females, and this seems to have been the case with pre-Homo erectus/ergaster ancestors. But at the same time the body size increase and tooth size decrease happen, we also see a reduction in sexual dimorphism in body size, strongly indicating a major change in social arrangements. The best two explanations for this may be a shift to a gibbon-like pattern of paired-off monogamous adults living more or less alone, or a human-like pattern of paired-off monogamous adults living in larger social groups.

It is an idea that would have caught on. It would have selected for more nuanced communication, and may thus have facilitated the origin of what we now know of as human language and symbolic processing.

So when you are eating your Thanksgiving dinner this year, most of which will be cooked, look around at the people at the table and, briefly, imagine them to be chimps. Then go back to your meal and try to put all those thoughts aside…

Penis Size: Does it matter and why?

A study just published in the Proceedings of the National Academy of Sciences explores the question of penis size and female preference in humans. The study involved making a set of 3D models of human males of various relative body sizes, and fitting them out with various size flaccid penises. These were shown to a sample of Australian women to get their reactions.

ResearchBlogging.orgThe assumption of this study is that at some time in the past humans did not wear clothing, so that information about penis size in men would be available to women who could observe flaccid penises and then choose sexual partners. That assumption is limited, perhaps flawed, in at least three ways:

1) We have no idea when men started to cover their penises on a regular basis. Ethnographically, there are very few cultures where men walk around with exposed penises, though there are several cultures in which men attempt to highlight and perhaps exaggerate the sizes of their mating equipment using various techniques. Since foraging people around the world, who stand in as models for the human “paleolithic,” often cover both male and female groin areas, it stands to reason that the practicer of covering up is old, even if it has not always been practiced. Archaeological evidence of early Homo (African and Asian Homo erectus/ergaster) strongly suggest that our ancestors, well before they became Homo sapiens, lived in a fairly wide range of habitats suggesting but not proving that clothing was developed as far back as just under 2 million years ago. If there was a period of universal exposure of the entire body, it may well have been much earlier than the evolution of anything looking like modern human culture and mating systems.

2) It is highly unlikely that human or pre/proto-human females would determine mating preference on the sole or primary basis of the details of the experience of copulation, assuming that some degree of paternal investment in offspring or the female herself was important. A better model of human mating suggests that females would look for a wide range of features, mostly behavioral, in long term male partners, and these longer term relationships would have more of an effect on selection (for a particular size penis) than a single variable.

3) There is not strong reason to believe that if females were interested in penis size as a factor in copulation that they would use flaccid penises to make assessments. The correlation between erect penis size and flaccid penis size is poor. In addition to this, in a social group in which no one wears clothing, other sources of information about erect penises would certainly be available. Penises would be erect at random times now and then, and in a social system where females make short term decisions about copulation, there would certainly be long term availability of information via the usual linguistic channels, after the evolution of language or proto-language, which would presumably be early(ish) in human evolution.

However, given these caveats, it may be reasonable to carry out the experiment reported in this paper because, well, why not?

The researchers note that human flaccid (visible) penis size is notably larger than that of our relatives, the great apes. This suggests that visual evaluation of penises was a selective force in human evolution. From the abstract of the paper:

Compelling evidence from many animal taxa indicates that male genitalia are often under postcopulatory sexual selection for characteristics that increase a male’s relative fertilization success. There could, however, also be direct precopulatory female mate choice based on male genital traits. Before clothing, the nonretractable human penis would have been conspicuous to potential mates. This observation has generated suggestions that human penis size partly evolved because of female choice. Here we show, based upon female assessment of digitally projected life-size, computer-generated images, that penis size interacts with body shape and height to determine male sexual attractiveness. Positive linear selection was detected for penis size, but the marginal increase in attractiveness eventually declined with greater penis size (i.e., quadratic selection). Penis size had a stronger effect on attractiveness in taller men than in shorter men. There was a similar increase in the positive effect of penis size on attractiveness with a more masculine body shape (i.e., greater shoulder-to-hip ratio). Surprisingly, larger penis size and greater height had almost equivalent positive effects on male attractiveness. Our results support the hypothesis that female mate choice could have driven the evolution of larger penises in humans. More broadly, our results show that precopulatory sexual selection can play a role in the evolution of genital traits.

What have we learned from this study? Perhaps, mainly, something about the reaction a certain subset of Australian women have to male penis size. However, we can also guess that human sexuality, including details such as this, are a product of our rather complex and difficult to parse culture. I am uncomfortable linking these results to either the behavior of paleolithic humans or to a model of sexual selection, given that human sexuality today is so diverse and clearly constructed from exposure to enculturation and lived experience. Is this scientific evidence that when people say things like “size does not matter” or “it’s how you use it that counts,” they are kidding? Perhaps. In Australia. But probably not.

To me, a more interesting study would look at biological and cultural variations in the relationship between flaccid penis size and erect penis size, and how information about these things would be made available in different normative cultural settings. For instance, I would predict that if penis size matters in relation to either female mate choice or male-male competition, this relationship would be strong (and flaccid penises generally larger) in societies where men don’t cover up, but uncorrelated (with little selection on flaccid penis size) in societies where men do cover up.

See also this at The Stochastic Scientist.

Mautz, B., Wong, B., Peters, R., & Jennions, M. (2013). Penis size interacts with body shape and height to influence male attractiveness Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1219361110

How Do You Get Sexual Orientation and Gender in Humans?

Humans appear to have a great deal of variation in sexual orientation, in what is often referred to as “gender” and in adult behavior generally. When convenient, people will point to “genes” as the “cause” of any particular subset of this diversity (or all of it). When convenient, people will point to “culture” as the “cause” of … whatever. The “real” story is more complicated, less clear, and very interesting. And, starting now, I promise to stop using so many “scare” quotes.

Continue reading How Do You Get Sexual Orientation and Gender in Humans?

The Kiss

Valentine’s Day is coming up, so it is time to think about kissing. Pursuant to this, Sheril Kirshenbaum, author of “The Science of Kissing,” has made the Kindle version of her excellent book available at a discounted price through February 18th. The book is here: The Science of Kissing: What Our Lips Are Telling Us. (Sheril is also the co-author of Unscientific America: How Scientific Illiteracy Threatens our Future.)

The Science of Kissing Book Cover
The Science of Kissing
Further details as well as a video (don’t worry, it’s work safe) are here, on Sheril’s site.

Also pursuant to Valentine’s Day, I thought it appropriate to repost my original review of Sheril’s book which comes with a story about a kiss:

The Kiss

I went out with a friend. We were both between relationships, and we both knew somehow that this was a date though it was never called a date. And we had a perfectly good time: Good food, good conversation, good drinks. She drove.

When it came time to go home, she drove me to the urban neighborhood I lived in and parked on the street near my house. As we were saying our good-byes, she enigmatically unhooked her seat belt. I wondered why. Then, I discovered that she wanted the freedom of movement to lean across the console and give me a kiss. It was a good kiss. It was actually a series of good kisses, and it went on for a while.

And suddenly, there was a loud rapping on the window of the car. We stopped kissing and that’s when we noticed that we had steamed up the windows a bit. So I cracked the window on which the rapping had occurred and there was a policeman staring in with his flashlight.
Continue reading The Kiss

Pornography Actresses: Testing the Damaged Goods Hypothesis

A study has just come out in the Journal of Sex Research comparing various psychological and lifestyle measures of women who act in pornographic films with matched sets of women who do not.

ResearchBlogging.orgThere is a pretty clear association between negative attitudes towards pornography and negative assessments of the quality of life for actresses in the pornography genre. Studies have shown that those who regarded pornography as harmful to society also believed that those acting in the films must not like their work. Studies have also shown that people tend to believe that porn stars have sexual and physical abuse in their backgrounds at a higher rate than the general population. Conversely, people who have more positive attitudes towards pornography also seem to have a more positive attitude about porn actresses. As a whole, the research that involved asking people what they thought about pornography and those who participated on the stage in making it painted a picture that has become known as the “Damaged Goods Hypothesis.”

The damaged goods hypothesis posits that female performers in the adult entertainment industry have higher rates of childhood sexual abuse (CSA), psychological problems, and drug use compared to the typical woman.

The purpose of the study at hand was to test this hypothesis. Among the numerous data collected for each participant, the following especially salient questions were asked: Continue reading Pornography Actresses: Testing the Damaged Goods Hypothesis

Falsehoods: Human Universals

There are human universals. There, I said it. Now give me about a half hour to explain why this is both correct and a Falsehood. But first, some background and definition.
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What is the most important human adaptation?

Human infants require more care than they should, if we form our expectations based on closely related species (apes, and more generally, Old World simian primates). It has been said that humans are born three months early. This is not accurate. It was thought that our body size predicted a 12 month gestation, and some suggested that Neanderthals would have had such, but this research conclusion has been set aside based on new analysis. But it is still true that developmentally, human children do not reach a stage of development that allows some degree of self care for a very long time compared to apes. The actual sequence of development is not directly comparable: It is not the case that after a certain amount of time humans reach a specific stage reached earlier in the lifecycle by Chimpanzees, as the differences are more complicated than that. For the present purposes, we can characterize the human condition for early development like this: Human babies are more helpless in more ways and for longer than comparable ape babies.
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Why do women shop and men hunt?

Or, when the hunting season is closed, watch teh game (the guys), or when there are no sales, admire each other’s shoes (the gals)?

This is, of course, a parody of the sociobiological, or in modern parlance, the “evolutionary psychology” argument linking behaviors that evolved in our species during the long slog known as The Pleistocene with today’s behavior in the modern predator-free food-rich world. And, it is a very sound argument. If, by “sound” you mean “sounds good unless you listen really hard.”

I list this argument among the falsehoods, but really, this is a category of argument with numerous little sub-arguments, and one about which I could write as many blog posts as I have fingers and toes, which means, at least twenty. (Apparently there was some pentaldactylsim in my ancestry, and I must admit that I’ll never really know what they cut off when I was born, if anything.)

Before going into this discussion I think it is wise, if against my nature, to tell you what the outcome will be: There is not a good argument to be found in the realm of behavioral biology for why American Women shop while their husbands sit on the bench in the mall outside the women’s fashion store fantasizing about a larger TV on which to watch the game. At the same time, there is a good argument to be made that men and women should have different hard wired behavioral proclivities, if there are any hard wired behavioral proclivities in our species. And, I’m afraid, the validity from an individual’s perspective of the various arguments that men and women are genetically programmed to be different (in ways that make biological sense) is normally determined by the background and politics of the observer and not the science. I am trained in behavioral biology, I was taught by the leading sociobiologists, I’ve carried out research in this area, and I was even present, somewhat admiringly, at the very birth of Evolutionary Psychology, in Room 14A in the Peabody Museum at Harvard, in the 1980s. So, if anyone is going to be a supporter of evolutionary psychology, it’s me.

But I’m not. Let me ‘splain….
Continue reading Why do women shop and men hunt?