Monthly Archives: February 2014

Sexy Symmetry

My recent post on attraction was, by a large margin, my most popular post yet. As this is an area of expertise for me, I’m happy to write more on the topic. This post will discuss in greater detail an aspect of attractiveness that I touched on briefly before: symmetry.

Leonardo da Vinci’s “Vitruvian Man” exhibits high symmetry. Image from

The human body is designed to have external symmetry. There are certain parts of us that are designed to be asymmetric — e.g. the brain, most of our organs — but for the sake of this conversation, “symmetry” refers to the parts of us that are supposed to be symmetrical. Evolution built our bodies to be perfectly symmetrical on the outside, and there is good reason for this. For example, if our legs were not the same length or equally-muscled, walking around would be inefficient. Overall, our symmetry allows us to have mechanical balance in our bodies, and to interact with our surroundings with both sides of our body (handedness not withstanding).

That we end up basically symmetrical as adults is remarkable. When our bodies grow, they do so by the individual cells in our body dividing, growing, differentiating, and stacking up to form our limbs and organs. This process takes place over one and a half decades, and involves roughly 10 trillion cells in the end product (not counting the trillions that die along the way). A symmetrical body therefore has roughly the same number of cells in the same locations on each half of the body. This is like two cars with blind-folded drivers traveling perfectly parallel and at the same speed for hundreds of miles. But our bodies do not turn out with perfect symmetry. Developmental perturbations, such as injury or disease, can disrupt the intended course of growth. If one of the drivers in my example runs over a small rock, his course and speed will be altered slightly, and he will deviate from the other driver. The number and intensity of these perturbations will determine how different the direction and location of the two drivers will be at any given point.

Thus body symmetry is an indicator of what we biologists call “phenotypic quality.” This is just a fancy way of saying how well-built a body is. Having higher symmetry means that you were able to prevent your body from being disrupted by developmental perturbations. Indeed, people with higher symmetry have higher intelligence, faster reaction time, are more masculine (if male) or more feminine (if female), are healthier, have better mental health, better mental acuity later in life, have higher sperm quality (only in men, obviously), and have better athletic ability. This is not a comprehensive list. The relationship between symmetry and these traits is sometimes small, but there is a clear relationship overall. Symmetry does not cause these traits, but symmetry is related to these traits because they all result from a body that is well-built.

And this is why more symmetrical people are more attractive.

Success in natural selection is about leaving more surviving offspring than other members of your species. If your offspring are put together better than the offspring of other individuals, they will survive better and ultimately leave more surviving offspring themselves. Choosing mates with high phenotypic quality will increase the chance that the chooser’s offspring will have high phenotypic quality. Resistance to disease is an aspect of phenotypic quality that is of particular importance in a mate. Remember from my post on sexual reproduction that an interest in producing disease-resistant offspring is a primary reason why sexual reproduction evolved in the first place.

For this reason, humans are not alone in our preference for symmetrical mates. This preference has been found in many species of birds, fish, insects, spiders, and even plants. The preference for symmetrical mates probably evolved shortly after sexual reproduction did. In my previous post, I list attraction to symmetry as a species-typic trait, but it is typical of way more species than just humans. An interest in high quality mates is integral to sexual reproduction.


The female peahen prefers peacocks that have more symmetrical “eyes” in their tails. This one looks pretty good. Image from

Symmetry is important, but it is only one trait of many that are involved in attractiveness in humans. As I write more about attraction, I hope the ways in which these traits interact will become more clear.


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You’re Doing it Wrong, part 1: The Naturalistic Fallacy

This is the first installment of what I hope will be a series of posts (probably not consecutive) on logical fallacies. Logical fallacies are a category of arguments that are never valid — they are incorrect ways of thinking. Today’s logical fallacy is the naturalistic fallacy (also called the “appeal to nature” or “Hume’s Guillotine”), which incorrectly equates what is true in the natural world with what is morally correct. 

Consider the following statements:

Eating meat is okay because humans evolved to eat meat.

Warfare is wrong because other animals don’t do it. (Some animals actually do engage in warfare, but people make this argument nonetheless.)

Homosexuality is okay because homosexuality is present in other animals.

Each of these arguments attempts to justify or condemn a moral claim based on what is or is not observed in nature. The first part of each statement (before the “because”) is an ideological claim and the second part of each statement (after the “because) is an empirical claim. Each part of each argument is valid as it is, and the opposite moral claims would also be equally valid. They become fallacious, however, when the two sides are associated. Science can only tell us what exists or what is true about the natural world — it absolutely cannot give us moral guidance.

No matter what your ideology is, you can find facts about nature that are consistent with your morals and facts about nature that are inconsistent with your morals. Some people believe that eating meat is wrong and almost everyone agrees that rape, murder and slavery are wrong. But these moral stances cannot be gained from nature, and neither can the opposite stances (that eating meat, slavery, rape and murder are morally sanctioned). Many animals eat other animals (e.g. dolphins, sharks, lions, wolves) and many don’t (e.g. cows, giraffes, horses, snails). Many animals rape (e.g. chimpanzees, chickens, orangutans, sea turtles). Some ants take slaves. Chimpanzees, langur monkeys and many other animals kill members of their own species. Nature is, as Alfred, Lord Tennyson said, “red in tooth and claw.”

There are a variety of ways the naturalistic fallacy can manifest:

1) People refuse to accept empirical findings because they are ideologically opposed to the outcome.

One time, I was explaining to someone the phenomenon that animals may kill some of their offspring when they cannot feed them all. She refused to believe it on the grounds that animals are more moral than humans.

2) People assign moral beliefs to a researcher based on the researcher’s findings.

For example, people sometimes claim that scientists studying sex- or race-differences are sexist or racist. For ideological reasons, some people like to believe that there are no differences between the sexes or among the races. When research threatens this, some people assume that the researchers making the discoveries are morally endorsing the findings. Interestingly, people don’t make this type of claim about medical researchers. When someone discovers a new form of childhood cancer, he or she is not accused of endorsing childhood cancer. Rather, it is seen as a previously unknown problem that we can choose to fix if our ideology drives us to do so. All scientific discoveries are morally neutral, and cannot threaten anyone’s ideology. Science tells us what is, not what we should do about anything, and the same goes for the scientists making discoveries. Things that are objectively true are true no matter who discovers it or what their own ideology is.

3) People claim that something is or is not moral because it does or does not exist in nature.

Many people who oppose homosexuality or gay marriage argue for their side by claiming that other animals are not homosexual. Many people who accept homosexuality and gay marriage argue for their side by pointing out that other animals ARE homosexual. Only the latter claim about non-human homosexuality is true, but both statements make the naturalistic fallacy. Science can tell us that homosexuality exists and why it exists and in what species, but it cannot tell us how we should feel about it.

There are two ways that science can interact with morality. First, science can study morality as a natural phenomenon, as some of my colleagues do. What parts of the brain are involved in morality? How is morality transmitted as an aspect of culture? Why is morality different in different areas? When these topics are studied by scientists, scientists are still not endorsing any given morality. They are not making a claim about which moral system is superior to another. They are just reporting on it as they would any other aspect of nature. (Note: humans and our morality are a part of nature.)

The second area is to determine how best to achieve one’s moral goals. For example, many people are morally opposed to war and are interested in reducing or stopping it. Science cannot justify this belief, but it can help you to understand the problem you are trying to solve and help you develop action plans for solving it. Some colleagues of mine have found that exposure to pathogens increase warfare. This finding in itself does not, of course, advocate a particular action. But if you are interested in reducing war, you could use this information to help. Conversely, if you are interested in increasing war, you could also use this information.

None of this is to say that you cannot have opinions or argue about morality — just that science cannot justify any particular moral or ideological stance.


See also:

You’re doing it wrong, part 2: Post Hoc Ergo Propter Hoc


Have a topic that you want me to cover? Let me know in the comments section.

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Hot or Not

With Valentine’s Day coming up, I thought I’d do a post about the science of attraction. The purpose of this post is not to summarize everything that we know about what makes people attractive to one another, but to explain how we think about it.

First, it is important to realize that attractiveness is not a single trait. Attractiveness is what we call an “emergent property” of many different traits — that is, people may possess many different traits that all add up to making them however attractive they are.

Attractiveness may be broken down broadly into two divisions: physical attractiveness and non-physical attractiveness. Physical attractiveness is the aspects of a person’s body that you find attractive (hands, feet, face, whatever else you’re into), and non-physical attractiveness are the personality, values and social traits of a person.

Another way of breaking down attractiveness is into how broad the preference is for certain traits. Traits may be typical of humans as a species, typical of individual cultures, or individual preferences.

Species-Typic Attractiveness

These are the traits that humans, as a species, tend to find attractive in one another. We can talk about this the same way we talk about any other trait in any other species. For example, the American lobster (Homarus americanus) is usually a reddish-brown color, but about one in two million is instead a brilliant blue color. The presence of the blue lobsters does not change the fact that the typical color of the species is reddish-brown. It just means that there are exceptions to the norm. In humans, the most basic rule of attraction is that men are attracted to women, and women are attracted to men. This is the species-typic trait. But a few percent of humans are either attracted to people of the same sex, to people of any sex, or to no one at all. The existence of these traits — which are far more common than blue lobsters — does not change what is typical of the species, nor are people with these traits any less human. We can tell when an aspect of attractiveness is species-typic when it is common across cultures and throughout time.

The following are examples of other traits that humans tend to find attractive as a species:

Symmetry is a trait that is attractive not only to humans but to many other species. People of both sexes are more attractive when they have more symmetrical faces and bodies. Denzel Washington, people magazine’s sexiest man of 1996, was once found to have an almost perfectly symmetrical face.

Examining the symmetry of Denzel Washington. Image from

Men are most attracted to women who have a low “waist to hip ratio” or a waist that is narrower than the hips. This is true of people across cultures and time. Waist to hip ratio (or WHR) is calculated by dividing the circumference of the waist by the circumference of the hips. Models and actresses commonly have a WHR of 0.65-0.75.  

Marilyn Monroe’s WHR was 0.63. Image from 

Conversely, women typically find men more attractive who have broad shoulders and relatively narrow hips.

Henry Cavill (as Superman) has an impressive shoulder-to-hips ratio. Image from


Men and women differ in the size of the lower face, and this difference is important in attractiveness. Women with a smaller lower face (relative to the size of the upper face) and men with a larger lower face (relative to the upper face) are more attractive. This is fairly hard to imagine from my description, but the following pictures of Mila Kunis and Nikolaj Coster-Waldau should make it more clear. Pay close attention to the shape of the jaw, the width of the jaw, and the distance from the chin to the nose:

Nikolaj Coster-Waldau. Image from

Mila Kunis. Image from

The most universally attractive non-physical trait is niceness. People of both sexes all over the world prefer romantic partners who are nice to them. This probably isn’t a surprise to anyone, but it has been confirmed by science.

Culture-specific Traits

Fashions for clothing, hair styles, facial hair, makeup, and jewelry are vastly different throughout time and space. High heels were originally a mens’ fashion that was later adopted by women. Nowadays a man wearing them is generally considered odd at best. Men used to show their wealth by wearing lots of necklaces and bracelets and rings. Men with wealth are still generally considered to be attractive, but modern, western conventions say “no” to all of the jewelry. Beards were in fashion when my father was young, then they were out for a few decades, now they’re coming back again. The perms of the 80s are mercifully gone, hopefully never to return.

This was all considered attractive in the 80s. Image from

This was all considered attractive in the 80s. Image from

Modern fashion often borrows things from fashion of the past, but completely adopting the style of past decades might be a little bit weird. Being weird typically makes one less attractive to others, unless you are in a culture that values being weird.

Some cultures find people more attractive when they are slim and some cultures like full figures.

The human population is the largest it has ever been, and the internet allows people to come together and form new cultures at a rate that has never been seen before. Adopting the conventions of a culture tends to make one more attractive to other members of the culture.

The “bagel head” trend gained limited popularity in Japan a few years ago. Image from

There is limitless variation to what individual cultures may find attractive, but there are no cultures that go against species-typic attractiveness by more than a little bit.

Individual Differences

Individual differences can cover both things that have been studied as well as the wide spectrum of unique things that people find attractive about one another.

People tend to like other people who are like them. This is called “assortative mating.” Political, moral and religious values can be a very important part of this. Liberals tend to date and marry liberals, Muslims tend to date and marry Muslims, vegans tend to date and marry vegans. The same goes for personality traits and even physical traits. There are obviously exceptions, but there is a clear trend.

How we feel about a person’s personality and behavior can influence how physically attractive we find them, and vise versa. If you have a negative experience with a person who is otherwise physically attractive, you will find them less physically attractive or even physically repulsive. The opposite is true as well.

Opposites don’t usually attract, but there are notable cases when they do. The most interesting case is probably the region of the immune system called the “major histocompatibility complex” or MHC. Humans and other vertebrates prefer mates who have different genes than their own in this region.

And sometimes there are just oddities that are unique to the individual. A friend of mine in college really liked guys wearing brown pants. There may not be anything substantial to explain about that.

This is just a primer on how scientists think about attraction. The few traits that I list are just examples and are by no means an exhaustive list of what we know (and there is still an enormous amount that we do not know). Of the things that I discuss, there are many nuances that I could not discuss for length. Indeed, whole books have been written on the topic. For more on this topic, see this book and this book and others. I plan to write more on this in the future, as well.

The point of this is not to tell anyone what they should or shouldn’t be attracted to. Everyone is different and the many aspects of attractiveness can interact in endless ways. People are attracted to whoever they are attracted to, even if it’s nobody.

What I hoped to accomplish here is to share a framework for how to think about the topic. The news is full of reports on new findings about human attraction. I hope it will be easier to put these studies into context after reading this post.

And try out this line tonight:

“Hey baby, I’m heterozygous at my MHC loci.”

Have a topic that you want me to cover? Let me know in the comments section.

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Only a Theory

With the recent debate between Bill Nye and Ken Ham, there has been a resurgence of people claiming that “evolution is just a theory.” This is something that really needs to stop. 

In common usage, the word “theory” means a guess. You might have a theory about who will play in the super bowl next year or what will happen later this season on American Horror Story. In this respect, your “theory” is as good as anybody else’s. This is not what we mean when we say “theory” as scientists.

In science, our level of certainty about an idea can be summed up with three words: speculation, hypothesis and theory.

A speculation is the lowest level of certainty. When we suspect that something is true but we haven’t done enough research on the topic to be confident in it, we use this word. This is like having a puzzle that is 10% completed and making a guess about what the missing pieces look like.

When a scientist forms a hypothesis, it means that they have learned everything or nearly everything that humans know about a subject, and they are confident in making a claim about something that is unknown. This is like having a puzzle that is completed but for a couple of pieces and making a claim about what the remaining pieces look like. My students will often define “hypothesis” as “an educated guess.” This is technically true, but it is not a particularly good definition because it implies a larger degree of uncertainty than there really is. While a particular hypothesis may not turn out to be correct, a scientist is confident enough in its accuracy to spend years of their life and tens of thousands of dollars determining whether or not it is true.

In scientific parlance, the word “theory” is synonymous with “fact.” It is an idea for which we have a great deal of evidence. A hypothesis can graduate to a theory once enough evidence is collected in support of it. Having a new theory is like adding a piece to a puzzle. When we know a new piece of information, it give us more insight into the things we do not yet know. It allows us to form new hypotheses that we couldn’t have before. So when someone says “x is only a theory,” they are saying “x is only a fact,” which doesn’t make much sense.

For reference, here are some ideas that are considered theories by scientists: The earth is spherical. The earth orbits the sun. General Relativity. Some diseases are caused by microorganisms. Plate tectonics. Electricity. The diversity of life on earth is the result of evolution by natural selection. Of these, the theory of evolution by natural selection is one of the most strongly supported — about as well-supported as the theory of the heliocentric solar system. It is a benchmark against which we can compare our certainty in other ideas. When something is supported as well as evolutionary theory, we know we’ve got it in the bag.

But when something is a theory, doesn’t that mean it hasn’t been proven? In a sense, yes. Outside of mathematics, scientists do not use the word “proof.” This convention is to constantly remind ourselves that we are not finished learning about things and that there is always a chance, however slim it may be, that we are wrong about something. Within mathematics, a statement can be proven within certain, carefully-established bounds. Some mathematically-proven statements are called “theorems.” Math aside, “theory” means as close to proven as we’re willing to say.

So what is a law? Isn’t a law higher than a theory? Not at all. As it is used, one could define it as a robust and generalizable observation. Unlike with a theory, there is no demand that we understand how it works. For example, Kleiber’s Law is the observation that the body size and metabolic rate of animals are related through the function y = x^3/4. There are hypotheses to explain why this is true, but these explanations are not a part of Kleiber’s law. Despite what many people think, theories do not graduate into laws once we get enough evidence for them. Rather, when we do have laws, we need to develop theories to explain them.

Whenever someone is trying to convince you that a particular scientific idea is wrong by claiming that it is “only a theory,” they are either deliberately trying to mislead you, or they are profoundly ignorant of science. These are the only two options.


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Darwin Didn’t

To commemorate Darwin Day (February 12), I thought I’d talk a little bit about Charles Darwin (1809-1882); perhaps the most important biologist ever to live. More to the point, I’d like to dispel the myth that he discovered evolution.

Charles Darwin. Image from

Evolution is the phenomenon that species, or individual populations of species, change over time. A more textbook definition of evolution is “a change in the frequency of traits or genes in a population over time.” The truth is that many scientists before Charles Darwin knew that life had changed over time. They knew about the fossil record and about selective breeding of animals to produce certain traits, and could see rather plainly that evolution had occurred.

Erasmus Darwin (1731-1802), the grandfather of Charles, dabbled in evolutionary theory himself. Convinced as he was that evolution was real (like many intellectuals of the day), he had the motto “e conchis omnia” (“everything from seashells”) painted on the side of his carriage. This reflected a primitive understanding of evolution that all life evolved from simpler forms. All life did, of course, all evolve from more primitive life, but it is not all descended from shellfish.

Painting of Erasmus Darwin by Joseph Wright. Image from

Jean-Baptiste Lamarck (1744-1829) was an early evolutionary theorist who is perhaps most famous for getting it wrong. He knew about evolution, but he didn’t know how it happened. His hypothesis for evolution was the inheritance of acquired traits. For example, if a naturally slight person lifts a lot of weights and gets very strong, his or her children would inherit those big muscles. In evolutionary terms, if a horse is constantly stretching its neck up to reach high branches, its offspring will inherit a slightly longer neck. After many generations the descendants of the original horse have evolved into giraffes. This was a good try, but it turned out not to be true.

Portait of Jean-Baptiste Lamarck by Jules Pizzetta. Image from

My point here is not to summarize the history of evolutionary thought, which goes back as far as the ancient Greeks, but just to show that many people were pretty sure of evolution before Charles Darwin came along. Charles Darwin’s big accomplishment was not discovering that evolution happens, but discovering the primary mechanism through which it occurs: natural selection. This stands today as one of the most important discoveries ever made.

Natural selection is the phenomenon that organisms which are better suited to their environment than other members of their species (and members of other species, as well) will survive better and reproduce more. They will pass these traits (which are based on genes) that allow them to succeed on to their offspring, and these traits will increase in frequency over time as a result. For example, the Thompson’s Gazelle is one of the fastest land animals in the world. Slower gazelles will be the first to become dinner for cheetahs. Faster gazelles will survive better and therefore leave more surviving offspring in subsequent generations. The offspring of the fastest gazelles are likely to be fast themselves because aspects of speed will be inherited from their parents. This results in a change in the frequency of traits and genes in a population/species over time. There are two key differences between this and Lamarck’s idea: First is where the variation comes from — Lamarck believed that the variation in traits was acquired, whereas natural selection relies on existing variation based on inherited genes. Second is that Lamarck did not comment on differences in survival or reproduction, which Darwin’s idea rests heavily on.

It is important to make the distinction between evolution and natural selection, because natural selection is only one way that evolution can occur, albeit the primary mechanism. Broadly speaking, the other mechanism of evolution is called “genetic drift.” Genetic drift is any change in the frequency of traits or genes in a population that is not due to natural selection — rather, it is due to random chance. For example, a natural disaster such as a meteor strike could kill all of the organisms in a particular area. If, through random chance, there happened to be more individuals with, for example, longer fur in the area hit by the meteor, then the population has changed — the frequency of the short-fur trait has increased. Unlike natural selection, survivorship is due to random chance as opposed to favorable traits. Also unlike natural selection, this change could just as easily have gone in the opposite direction, and the same change will not persist in subsequent generations. Genetic drift cannot produce complex traits the way natural selection can. When we talk about evolution, we are almost always talking about natural selection, but it is important to make the distinction.

In 1859, Charles Darwin published his famous book, On the Origin of Species — book of over a thousand pages which he described as “an abstract” for his opus magnum, which, sadly, he never finished. Darwin’s work was meticulous and, as a result, biologists and other scientists around the world quickly realized that he was right. The theory of evolution by natural selection became the unifying theory of biology, prompting Theodosius Dobzhansky (1900-1975) to note that, “nothing in biology makes sense except in the light of evolution [by natural selection].”

I will leave you with what has become the most famous passage from On the Origin of Species:

“It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”

– Charles Darwin, from “On the Origin of Species” first edition, 1859.

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