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Sexual Rhythms

The swaying of breasts, the menstrual cycle, and hip thrusts are just a few of the important oscillations in our sex lives. Physics provides a simple and powerful description of rhythmic motion and cycles, and can help you get the most out of oscillations, from choosing the best bed for your sexual pleasure, to enjoying and exploiting the natural rhythms of your body parts during sex.

Listen to the podcast with roboreaders Audrey and Paul.

Seasons shift through the course of a year, the ocean tides ebb and flow every day, and your mood may swing with the periodic changes in the chemicals in your brain, but the most common types of oscillators are mechanical - a tree bending back and forth in a breeze, a string bowed on a violin, or a couple making love.

Mechanical oscillators work by transferring energy between two forms - kinetic and potential. Anything that moves has kinetic energy. Stored energy is potential, as in the case of a ball poised to roll down a mountain. The energy in oscillators is sometimes kinetic, sometimes potential, and usually a bit of each.

When a playground swing moves back and forth it's briefly stationary at the highest point in its arc. For an instant it has no kinetic energy. Because it is higher above the ground than at other times in its motion, it has a maximum amount of potential energy, just like a ball on the verge of rolling down a hill. When the swing descends, it speeds up as its potential energy is converted to kinetic. The swing is moving fastest, with the most kinetic and least potential energy, at the lowest point of the arc. As it swings up in the other direction, the kinetic energy is converted back into potential.

Similarly, when you make love on your bed - at least when one partner is bouncing up and down on top - you rhythmically compress the mattress springs. The springs have lots of potential energy when they are compressed, but as the springs extend and push you upward, the potential energy is converted into the kinetic energy of your moving bodies.

The rate that the energy flows back and forth in an oscillator is its resonance frequency. The frequency of an oscillator is measured in hertz, which is the number of oscillations in a second. A clock ticks at one hertz , or once per second; your heart can beat at two hertz or more during heavy exercise and sex; and middle C on a piano is a 440 262 hertz vibration in air, which we hear as a musical note.

All oscillators have at least one resonance frequency. Many, such as violin strings, have several resonances. The distinctive sound of an instrument has a lot to do with the many resonances that are produced along with every note; it's the combination of resonances that ensures that a violin and a piano produce rich and distinct sounds even when they are playing the same note.

One resonance, however, is usually more important than the rest - it's the fundamental resonance. In the case of a musical instrument, the fundamental is the note a musician is playing. When a violinist chooses to play middle C, for example, he or she presses on a string to ensure that its fundamental resonance occurs at the 440 hertz frequency we identify with the note. Other, lesser resonances are called harmonics.

Lovers on a bed form an oscillator with a fundamental resonance and a spectrum of harmonics. Just as a gifted musician can make sweet music by adroitly manipulating an instrument's resonances, lovers can add to their resonant bliss in the bedroom by understanding and controlling their bed's oscillations.

When you sit on a bed, you'll sink into the mattress until you reach an equilibrium position. At that point, the force of gravity pulling you down is balanced by the force of the bed springs pushing you up. If you start to bounce up and down, you'll find that there's a certain frequency that allows you to get a big steady bounce. That's the bed's fundamental resonance. Most beds resonate at frequencies of a few hertz.

By rhythmically bouncing on the bed, you're doing what physicists call driving the oscillator. It's easiest to drive an oscillator at its resonance frequency. At frequencies just below or above resonance, it takes much more force to get a big bounce. If you start out very slowly, you'll probably end up oscillating well below your bed's fundamental resonance and won't bounce on the bed much at all.

Increasing your speed can bring you into resonance, allowing you to achieve large bounces with seemingly little effort. Once you're moving too quickly , you may pass the resonance. As a result, you'll end up working against the bed's rhythmic sweet spot. If that happens you will have to exert much more force to get a good bounce at the same time that you're trying to move quickly. The chances are, you'll be rapidly exhausted. If you stick close to the resonance, on the other hand, you get maximum motion for minimal energy input, which helps you keep going longer before you wear out.

Many things can affect a bed's dynamics, but it's the mattress' firmness that plays the greatest part in determining the resonance frequency. Firmer mattresses have higher resonances than soft mattresses. If you visit a mattress store, you can check this for yourself. Sit on several beds with different firmness and bounce on each. You'll find that the frequency varies from a very firm to very soft mattress.

Beds get their bounce from springs, and springs have resonance frequencies that depend both on their firmness, which physicists call the spring constant, and the mass on top of the bed. That means the resonance frequency will be different for you than it will be if you have someone in bed with you. In fact, if you and your partner are about the same weight, the resonance frequency will be roughly half two-thirds as fast with the two of you close together on the bed as it will be with just one of you. (To be more precise, it will be 1/(the square root of 2) or about 0.7 times slower.)

Determining a bed's resonant frequency is only part of the issue. After all, you may not be content within the confines of one rhythm. Fortunately, there's another factor that affects bed motions. Harmonic oscillators, and beds in particular, often include a certain amount of damping, which gives you a little more leeway in choosing your own rhythms.

Shock absorbers in cars are a good example of damping. Car suspensions consist primarily of simple springs. When a car hits a bump, the springs allow the wheels to travel up or down relative to the car, maintaining contact with the road. If it weren't for shock absorbers, a car would continue to oscillate on its springs after hitting a speed bump or a pothole, leading to a nauseatingly bouncy ride. Shocks settle a car down quickly by dissipating the energy of the bounce. As a result, they reduce the resonance frequency and make the resonance less pronounced. If you push down on the bumper of a car with bad or missing shocks, you can easily get it to resonate and bounce dramatically. It is much more difficult to find the resonance frequency of a car with good shocks.

Damping has the same effect on a bed that shock absorbers have on a car - it will be harder to drive a resonance on a very damped bed, but at least you won't suffer the frustration that comes from trying to move at rates higher than resonance.

Although no commercial beds currently come with automotive-type shocks, padding in the mattress adds damping. Alternatively, you can add your own damping by spreading a thick comforter on the bed and making love on top of it. A few well-placed pillows under you can increase damping too.

The bottom line is this: if you want to use the resonance to your advantage and you like it fast, choose a firm bed; if you like it slow, go with a soft bed; and for maximum flexibility, buy a firm bed but keep a few soft comforters and pillows around to dampen the resonance to suit your mood.

If you have the soul of an experimental physicist, try making love on a trampoline, which has almost no damping and a powerful resonance. Then try it on a water bed, which also has little damping and strong resonance, as the water sloshes from one place to another, but at a much lower frequency than a trampoline. To round things out, make love on a squishy foam bed, like the Tempurpedic mattress, to experience lots of damping with very little resonance. It can be an exhausting and frustrating challenge.

Some people find that their favorite lover is a machine - specifically, their vibrator. Vibrators get their buzz from an electrically powered oscillator.

In battery-powered models, vibrations generally come from an electric motor attached to a rotating disk, with its weight placed off-center. The principle is the same thing that causes unbalanced washing machines - which are some, other, people's favorite lovers - > to buck violently when more of the laundry is on one side of the washer drum than the other. The faster the motor turns, the higher the vibrator frequency.

Many of the vibrators that plug into the wall generate oscillations with a different type of electric motor called a solenoid. Instead of spinning an unbalanced weight, electricity passing through a coil of wire forces a metal slug to vibrate rapidly back and forth. The speed of vibration in a plug-in vibrator is related to the 60 hertz oscillations of the electricity in wall sockets. They are usually more powerful than battery vibes. Unfortunately, their power comes at a price - they can only vibrate at the frequency of the electricity in the wall, at a multiple of the electrical frequency, or certain fractions of the wall frequency. Unlike battery powered vibes, which can run at a spectrum of speeds, most plug in vibrators have only one, two or three speed settings.

Ideally, vibrators would also come with adjustments to increase the strength of the vibrations independently of the speed, but that is not the case with any vibrators currently on the market. This is likely due to the fact that adding a power setting would complicate vibrator design, but you can always adjust the power you feel by changing how hard you press the vibrator against your body.

Vibrators have resonances just as beds and cars do. That means that increasing the speed of a continuously variable model can either increase the power of the vibrations or decrease them, depending on whether you are approaching or passing the resonance. As a rule, the power of the vibrations will slowly increase as you turn up the speed, then reach a maximum at the resonance frequency, and slowly fall as you continue to turn it up.

Fortunately, you can gain a bit more control over your vibe with damping, just as you can use damping to adjust a bed's dynamics. Changing how tightly you grip a vibrator and where you hold it will change the amount of damping, allowing you to modulate the speed and intensity. Inserting it into your vagina or anus will also change the speed as the soft tissue touching the vibrator absorbs energy. If you listen to the pitch as the vibrator moves in and out, you can hear the speed change. Alternatively, pressing the vibrator against a soft rubber or gel can also slow it down.

Some dildos have a cavity that allows you to insert vibrators into them. The softer and heavier the dildo, the more it will dampen the vibrations and slow the resonance. Many manufacturers of plug-in vibrators offer soft sleeves and attachments to allow you to dampen oscillations in the same way.

Damping is the reason that vibrators are more comfortable when used in the anus or vagina than on a hard penis or clitoris. A rigid clitoris or penis has much less damping than softer and more enveloping anal and vaginal tissue. The vibrational motion, and resulting energy, is transmitted at full intensity to sensitive nerves in a small area where the vibrator makes contact with the rigid tissue. In the vagina and anus, the energy is distributed to more tissue and nerves, which feels less intense.

Springs, like those in a mattress, are just one classic type of oscillator. Pendulums are another. A pendulum consists of a weight at the end of a rod or string. It will swing at a rate determined by its length, regardless of the amount of weight on the end. A long pendulum swings slowly, and a short pendulum swings quickly. It's easy to adjust a pendulum's frequency by changing its length. Old fashioned clock pendulums included adjustments for fine tuning the length of a pendulum depending on whether the clock ran fast or slow; if it ran too fast you could turn a screw to lengthen the pendulum, or turn it the other way to shorten it if the clock ran slow.

Much of sex involves motions that have the characteristics of a little of both pendulums and springs. From a physics point of view, a woman's breasts are a complicated combination of springs and pendulums. Depending on whether she is lying on her back, standing, or on her hands and knees, her breasts are more like one or the other, which can have a huge effect on how they move.

Take a woman on her hands and knees, for example. Her breasts will hang down and sway as a result of her motions. Because hanging breasts are similar to pendulums, they have resonances determined primarily by the length they extend from a woman's chest. The resilience of her skin and breast tissue will also have an effect, but for gentle motion, the pendulum-like aspects are most important. Breasts will naturally swing slower if they hang farther from the chest, and swing faster if they are more compact.

It's easy to tell when hanging breasts have reached their resonance frequency, because they will swing dramatically back and forth. Increasing the frequency of the forces driving her breasts will reduce the amplitude of their motion, as they pass the resonance frequency, until they stop moving altogether.

When a woman is on her back, her breasts will tend to resemble springs more than pendulums, and will resonate at a frequency that has more to do with their resilience and mass. Heavier breasts have lower resonance frequencies, and tauter breasts (with higher spring constants) have higher resonances.

Resonant frequencies of breasts vary dramatically from one woman to another, and will vary even in a specific woman as her breast size and tissue resilience changes over time, or as she changes position from standing to lying on her back to getting up on her hands and knees.

Resonances are also the reason some women need sports bras when they exercise. It can be painful if a woman moves with rhythms close to her breasts' resonance frequency because that is when they are moving the most. Sports bras solve the problem by compressing breasts and making them, in effect, more taut. This raises the resonance frequency, hopefully beyond the frequency of jogging and other repetitive motions. Depending on a woman's cup size and the design of the bra, it may just move the resonance frequency up enough to make one exercise comfortable while making another painful.

For instance, if a woman with large breasts finds that her resonance frequency comes at a slow jog, it's possible that she will experience less motion if she sprints at a rate that drives the breasts at frequencies above resonance. Essentially, speeding up changes the bounce to a jiggle. Potentially, if she were to wear a sports bra that makes jogging comfortable it could move the frequency up to the point that things get bouncing and painful when she sprints.

Penises too have natural frequencies, which can change depending on arousal. Although the first sports bra (according to one story of their origin) was made from a pair of modified jock straps, men do not usually need extra support to prevent the sorts of resonances that plague women's breasts during exercise. A hanging, flaccid penis and testicles form short pendulums with resonance frequencies well above the frequencies of nearly any athletic activity. All jock straps do is lift the testicles up and forward to keep them from being squashed between the thighs - resonances aren't usually an issue during exercise.

An erect penis, however is like a large mass on a spring attached at the pelvis - the larger and heavier the penis, the lower the resonance frequency - and it may well resonate at rates that would interfere with many sports, but a man with an erection probably isn't in the right state of mind for jogging, basketball or soccer anyway.

Some types of penis enlargement involve snipping the tendon that supports the penis, allowing it to extend farther from the pelvis. This can radically reduce the spring constant of the penis attachment, and lower the resonance frequency of the erect penis a great deal. It's hardly iron-clad evidence, but if you watch a porn movie, you may notice that two men with similar sized erections seem to have very different penile resonance frequencies. (Look for the motion when their penises are not being touched directly, but are being driven by some indirect motion, say shifting position on the bed or walking across a room in the nude.) It's possible that a fellow with a low frequency oscillation has had his penile tendon snipped.

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Bouncing beds, humming vibrators, and oscillating body parts are only a few of the ways that simple harmonic motion is important in the boudoir. In fact, your entire body is a kind of a simple harmonic oscillator. Moving from one position to another changes the portions of your body that come into play and affects the resulting resonances. The rhythm that feels natural with one partner on top is likely to be different from the rhythm when the other partner is on top, particularly if they are significantly different sizes.

Clearly, we've only touched on a fraction of the ways rhythms are important in sex. We hope it's enough to convince you to keep oscillations and resonance frequencies in mind, in order to help you enhance your sexual experience - whether you're choosing a bed, buying a vibrator, or searching for a new position.
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Why do we have sex? Part 3

In the competitive game of natural selection, the winning organism is the one whose DNA is most prevalent and longest lasting.

Listen to the podcast with roboreader Kate.

When a bacterium divides, the offspring carry the same DNA as the parent, as do the offspring in the third and fourth generations, and every generation after.

Sexually reproducing organisms lack this sort of genetic continuity. Your children will each carry genes that come in part from you and in part from their other parent. When your children breed, the portion of their DNA that you contributed will be further diluted. In the course of a few generations, there's not likely to be much of you left in your descendents. On the bright side, genetically speaking, at least your descendents and your species as a whole, can survive in the face of extreme stresses, while an asexually reproducing species could be wiped out easily. In fact, the scarcity of asexual species suggests that they hardly ever survive for long in nature.

I know what you're thinking. If two sexes are better than one in dealing with stress and mutations, wouldn't three be better still? The fact that there don't seem to be any suggests the answer is no.

Nevertheless, physicists have developed mathematical models of hypothetical creatures that breed in sets of three. (The researchers who wrote the papers examining the three-sex models, unfortunately, didn't bother to explain how these creatures would get their three-way groove on.)

Three-sex creatures and their offspring are triploid, with three complete sets of genes rather than the two of diploids like us, or the single genes of haploid amoebas and other simple animals and plants.

The numbers work out poorly for three-sexed creatures. For one thing, it's much more complex to get a reproductive trio together. The simple fact that it takes more parents to produce the next generation means that the population will grow slower than that of competing diploids just as diploids are out-bred by asexual haploids. Triploids, however, have one thing going for them - they are less susceptible to random mutations than haploids or diploids, thanks to even greater genetic redundancy than two gene animals like us. Unfortunately, they lag behind when it comes to adapting to other sorts of stress. Like so many cases of competition in nature, too much of a good thing turns out to be bad.

If triploid genes work in the same way ours do (which seems like a good guess) then each of the three genetic sequences has a genes for every trait, but it's the dominant one that wins out, or the trait ends up being a blend of all three. If the blue eye gene is dominant in a triploid population, the fact that each member of the population has three shots at getting a blue eye gene means that it is much more likely that everyone will have blue eyes.

It's easier to grasp the problem if we take it a few steps further. Imagine a population with ten sexes (and ten sets of genes per cell), and blue eye genetic dominance. Even if nine out of ten genes code for non-blue eyes, the one remaining blue eye gene wins. In cases of incomplete dominance and co-dominant genes, blending more and more versions of a certain trait leads to genetic uniformity instead of diversity, just as mixing more and more colors from a painter's palette results not in brilliant new colors, but to ever muddier shades of brown. While single-gene haploids have essentially no genetic diversity, increasing the numbers of complete genes in an organism beyond the two of diploids also leads to steadily decreasing diversity, which means less and less opportunity for evolutionary adaptation.

For most organisms, at least the larger and more structurally complex ones like humans, two-sex genetic diploids have an optimal combination of diversity, adaptability, reproductive efficiency, and resistance to genetic errors.

When humans rely solely on the tools nature provides us, we reproduce as most two-sex creatures do - a male provides semen, a female provides an ovum, and another generation begins. With a little medical and scientific intervention, however, we have lots more options.

Artificial insemination of course is one of the oldest and simplest alternatives to actual intercourse. Infertility treatments involving insemination in a Petrie dish are much like the external insemination practiced by fish and other aquatic and amphibious creatures.

While it is not triploid sex, when a woman serves as a surrogate mother for a fertilized ovum she is part of an interaction much like the three part male-female-female mating model.

Human cloning, however, is perhaps the most controversial method that may soon be among our potential reproductive options. Setting ethics aside, humans who reproduce via cloning would gain many of the asexual benefits that bacteria enjoy. Presuming that people who choose to clone are women who carry their own fetal clones in their wombs, and tend to have the same numbers of children over their breeding lifetime as other women, they would be able to increase their numbers much faster than sexual human couples could.

Men who opt to clone themselves need to seek out a woman to host the fetus in her womb, which means the process still takes a man and a woman. So that's really not an advantage numerically. In addition, the woman host would have to agree to waste precious reproductive time and effort to bear a child of no genetic relation to her. Surrogate mothers do that today for couples who can't, or choose not to, carry their own children to term, but surrogates usually require fee in exchange for the rented womb.

In a community where male-female couples and cloning women each choose to raise two children, the numbers of mating couples stays constant with each passing generation, but the numbers of clones double from one generation to the next. The clones' numbers could grow exponentially. Again, it's the males that are the reproductive liability in mating couples.

Other asexually reproducing creatures are highly susceptible to changing stresses in their environment, which favors sexual populations molded by natural selection. Humans, at least those living in more highly developed nations, tend to deal with stresses through controlling the environment and counteracting the stresses rather than through evolution.

If it's too cold, there's no need to evolve fur; we turn up the heat or put on a coat. If it's too hot, we turn up the air conditioning. We no longer adapt natural immunity to diseases; instead we develop vaccines, antibiotics, and prevention methods. As a result, people who are reasonably well suited to surviving in modern society, and who reproduce via cloning, would face few, if any, drawbacks from their asexual reproduction while gaining all the benefits.

Scientists have managed to clone many types of animals including cattle, cats, sheep, and monkeys. As of this writing, there have been no confirmed human clones produced from adult DNA. The complexity of cloning and the risks of severe birth defects mean it may be years before human cloning is perfected. But it will happen eventually.

If cloning ever becomes accepted practice, society could rapidly become populated primarily with women who reproduce by strolling down to the corner pharmacy to pick up the Acme Home Clone kit, rather than wasting time and energy looking for a male mate.

Clones won't evolve, so there is no reason for them to lose their sexual urges. Although they will have to learn to rely on lesbian sex to fulfill their needs, because men will eventually die out as sexually reproducing people lose ground to the clones.

When clones come to rule the world, sexual intercourse will be nothing more than a source of recreation, relaxation, and social bonding. Sex will be useless for procreation.

Of course, that's already true 99.99% of the time anyway.
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Why Do We Have Sex? Part 2

Although there are countless exceptions and variations when it comes to reproduction, there is one fundamental characteristic that typically distinguishes between sexual and asexual organisms - the structure of their genes.

Listen to the podcast by roboreader Kate

Most asexually reproducing plants and animals carry a single complete set of genes in their cells. In biological terms their genes are haploid, which is just a Latin word for simple.

Sexually reproducing organisms, on the other hand, are generally die-ploid, meaning they carry two complete sets of genes. Every one of your cells effectively has twice the genetic information it would take to make a single person. Your mother and father each contributed genes that encode the color of your hair and eyes, the size of your nose, the proportions of your limbs, and so on. Your mother's genes determine some of your traits, your father's genes determine others, and some of your traits are determined by both your mother's and father's genes.

Asexual reproduction has some distinct benefits. For one thing, it's fast. If a typical bacterium takes twenty minutes to split in two, it will have eight descendants in an hour, and sixty-four in two hours.

If there's sufficient food around to support a population explosion, a single germ dividing at this rate could boast more than a million offspring in seven hours, and over a billion in ten hours. It's what physicists call exponential growth.

Sexual creatures can grow their numbers exponentially as well, although not as fast as asexuals. If two populations of organisms, identical in every way except for their mode of reproduction, were to squeeze into the same ecological niche, the asexual population wins, at least in the short term.

Rabbits are the iconic example of animals that breed like, well, rabbits. Imagine a fertile valley that's settled by two colonies of ten rabbits each. One colony consists of five male and five female rabbits that reproduce once a year in the usual way. The other is a colony of ten asexual females that also reproduce once a year, but have no need of males (fortunately this is a purely hypothetical type of rabbit).

If every female rabbit can bear a litter of ten babies each season, then in one year the sexual rabbits increase their numbers by five litters - one litter from each female - resulting in fifty babies plus the ten original colony members, for a total of sixty sexual rabbits. The asexual colony has ten litters, one hundred babies, plus the original ten for a total of a hundred and ten rabbits.

Presumably, the amazons give birth only to females, while the sexual rabbits produce half male and half female babies. After a single breeding season, the females in the colony of asexual rabbits outnumber the sexual females by nearly four to one.

The all-female asexuals will swamp the sexual rabbits in a few generations. Even if there are limitations of food and water in the valley that keep the total number of rabbits in check, the asexuals efficient breeding scheme allows them to overrun the sexual rabbits in short order.

As you can see, males are the true liability in breeding populations - they eat food that could go to the girls, produce waste, and take up precious space in the colony. But they are of little help in increasing population numbers other than donating sperm, which asexuals can live without.

Why don't we see asexually reproducing rabbits, squirrels, rats, elephants, or humans in the real world? The answer lies in adaptation to stress. And we can thank males for that.

Asexual populations consist essentially of clones, with each child carrying exactly the same genetic material as its parent. If a new disease, parasite, or predator were to come along with a particular talent for attacking our asexual rabbits, the whole population could be rapidly decimated.

Sexual rabbits have a better chance of surviving in the face of stress thanks to the presence of the boys. In mating, the male and female of a species each contribute a portion of the offspring's genetic material, which means babies are always at least slightly different from their parents. Sex stirs the genetic pot, leading to combinations that may occasionally handle stresses better.

When a fox finds a valley full of bunnies, you might imagine that it eats the slowest ones first. All the asexual rabbits are equally swift because they're identical. If the fox can catch one it can catch them all.

Some of the sexual bunnies however, will be faster than others as a result of the variability that comes from male-female breeding. Pretty soon, the pressure of having a fox hanging around might lead to natural selection of fleet-footed bunnies. Of course, rabbits could deal with foxes in other ways - developing better camouflage, enhancing their wariness, or growing wickedly sharp claws. But in any case it's the sexual ones that have the potential of finding solutions, while the asexuals are doomed.

Once sexual rabbits have developed an adaptation to deal with a specific stress, you might wonder what is to prevent them from spontaneously changing reproductive tactics to become a new asexual super rabbit that can fend off a given type of threat.

Based on some physicists' models, the primary reason is that foxes, germs and parasites evolve as well. A rabbit that adapts to the stress of a certain fox causes stress for the fox by denying him food, which in turn leads to the evolution of better hunters, forcing rabbits to evolve further, and so forth. Populations of rabbits and foxes ebb and flow as each adapt to changes in the other, leading to long-term stability of predators and prey that is maintained by sexual mixing of each species' genetics.

As organisms evolve, they face lots of shifting stresses, which firmly establishes sexual reproduction as the procreation method among just about everything larger than an amoeba.

Even if there were no threat of predators, parasites or diseases, all life faces the risk of random genetic mutations. Mutations are changes that arise from errors that occur when DNA replicates, or from exposure to things like radiation and chemicals. Asexual organisms that have a single precious copy of their DNA are in deep trouble as errors accumulate. Sexual organisms gain protection through their genetic redundancy - if an error develops in a gene contributed from one of your parents, repair mechanisms in your DNA can use the genes from your other parent as a map for repairing the problem. Or the problem may be moot if a healthy gene is dominant over the flawed copy.

In short, sex provides multiple levels of genetic protection. It offers a route to adaptation through gene shuffling, ensures backup copies of genes are available, and keeps genes in good shape with DNA repair mechanisms.

Tune in next time for part three of, “Why Do We Have Sex.”

I'm Kate. Thanks for subscribing to The Physics of Sex podcast.
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Why Do We Have Sex? Part 1

If your answer is "to make babies," you're wrong. Sexual activity among humans has nothing to do with fertilization more than 99.99% of the time.

Listen to the podcast by roboreader Kate

Typical heterosexual couples make love an average of a hundred times a year. Assuming they keep up this pace most of their adult lives, they will end up having had sex as many as four thousand times.

In addition to sex with a partner, most people seek lots of sexual relief when they're all alone. Men typically learn to masturbate in adolescence and keep up the practice daily until their twenties. The pace usually slows down as men age and often dips when they enter sexual relationships, but most men will probably masturbate ten thousand times in fifty to sixty years of sexual activity. Altogether, the average man can expect to experience fifteen thousand or more orgasms over the course of his life.

Women start off masturbating at a similar age and frequency as men. Most masturbate daily until their late teens, but slow down when they reach their early twenties to about a third the rate of men. Still, they typically enjoy sexual stimulation, either alone or with a partner, for a lifetime total of five to ten thousand sexual experiences.

Despite all that sexual activity, the population in the US and most other highly industrialized nations is fairly stable. That is, there is roughly a single child born for each person in the country, which means that there is one successful pregnancy for every ten thousand sexual experiences.

Humans are unusual creatures in this regard, though hardly unique. Certain apes, dolphins and wolves are among the animals that use sexual interactions for things like pleasure, bonding, and establishing social structure. But intercourse for most other organisms is all about making babies.

Even though humans rarely have sex in order to get pregnant, it's primarily our genetic mandate to mate and bear young that is reflected in our sexual desire. Evolution ensures things that are good for the propagation of our genes bring us pleasure. For most people, and apparently many animals as well, the orgasm is the benchmark of pleasure. The fact that it produces the most enjoyable sensations and the strongest desires that we are likely to ever experience suggests that sex is effectively the most important thing we do in life, from an evolutionary point of view. Although we have found ways to separate intercourse from procreation, sexual ecstasy is nature's reward to us for continuing the species.

Sex between a male and a female, however, is not the only way for organisms to reproduce. For millions of years, when life was new on Earth, plants and animals got on just fine without intercourse. Most of them were simple single-celled organisms. At times when there were plentiful resources to keep some organism alive, rather than letting anything go to waste, or go to a competitor, the organism would simply duplicate its DNA and split into two identical copies of the original. Modern bacteria, amoebas, and many types of algae and molds continue the practice.

A look at the pros and cons of various types of reproduction can help explain why we mate the way we do instead of opting for asexual reproduction, or some other scheme altogether. For physicists who ponder reproductive strategies, the subject is similar to many other sorts of problems in physics where systems, in this case populations, naturally find the optimal solution to complex and competing demands.

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