Updated: Nov 14
If you’ve ever been in a relationship, then you don’t need to read about sex to know its complicated. But have you ever wondered why it’s so complicated? Read on, and get your honorary armchair degree in evolutionary psychology by the end…
1. The mystery that is sexual intercourse. What is the point of sex?
This question sounds so obvious that it doesn’t need answering, except for the scientists who study it. Sexual intercourse, from an evolutionary perspective, doesn’t seem to make much sense at all.
In fact, asexual reproduction is a much more efficient way to pass along genes to the next generation. Sex is also a recipe for extinction. Imagine two groups of animals living in the wild: one produces asexually (as in, on its own, without the need for a partner) and the other group reproduces via sex as we know it. The asexual group would quickly outnumber and out-reproduce the sexual group because all of them would be having babies. In normal sexual relationships, only the female can actually give birth.
From a biological and evolutionary perspective, sex appears to be a disaster. Here are some other reasons why scientists are so puzzled:
Sexual animals waste a lot of valuable time and resources attracting and competing for mates.
The act itself is also a distraction, as predators in the animal world can easily pick off breeding prey.
Sex doesn’t guarantee the ‘good’ genes will always pass on to the next generation. Sometimes ‘bad’ genes are stronger than the good ones.
But obviously sex, biologically speaking, must not be a disaster. Otherwise evolution wouldn’t have ‘selected’ it as the method of choice for reproduction.
What we think we know about sex
The theory of evolution, first proposed by Charles Darwin, follows that all living creatures on Earth slowly change over time. Darwin did not know anything about DNA and genetics, but he recognised how similar types of creatures living in slightly different locations had learned to adapt to the environmental differences.
It is now understood that all life on Earth is made up of DNA. Everything. Your body is made out of DNA, just like a mushroom is made up of DNA, and a jellyfish. DNA is like a set of instructions in your body, the blueprint that makes things happen. These instructions can be broken down into specific sets of instructions called genes (see above).
In 1976 a biologist called Richard Dawkins published a book called The Selfish Gene. In it, he described our bodies as ‘slaves’ to the genes. All the genes want to do is survive, and they survive by making copies of themselves in our children and grandchildren. The desire to have sex then, is nothing more than a set of instructions in our bodies telling us we must do so. That way, our genes cheat death. We may die, but the genes live on in our children, and so on.
So in short, one explanation for sex could be: we are all just vessels for our genes, a stepping-stone on their quest for immortality.
(By the way, Dawkins never said the genes were conscious and knew what they were doing. He was talking metaphorically.)
Asexual reproduction versus sexual intercourse
The genes in our bodies are not free-thinking creatures. They respond and change slowly to the environment. Sometimes mistakes are made — the ‘bad’ genes that we spoke of earlier — these genes can be disastrous. They carry with them instructions for illnesses and disabilities that can prevent sexual intercourse from every happening and lead to an early death.
It stands to reason that evolution would have selected a method of procreation that does away with bad genes. Indeed, this is one of the reasons why scientists think sexual intercourse is better than asexual reproduction.
Muller’s ratchet is the phenomenon that bad genes will build up over time in creatures that reproduce asexually. The idea is that, over time, there will be so many bad genes in asexual creatures that they will all die out. Normal sexual intercourse allows opportunities to avoid these bad genes in offspring by merging with good genes from a partner.
The trouble with Muller’s theory is that some asexual creatures have successfully reproduced for at least 70 million years. Such as the bdeloid rotifers (teeny tiny aquatic creatures). Meanwhile in the laboratory, scientists have genetically altered fruit flies, lizards, and algae to reproduce asexually, instead of sexually, and in some cases, the new asexual hybrids have performed even better than their sexual counterparts.
And considering the main advantage of sex is (supposed to be) to flush out bad genes, some creatures that suffer the worst mutations don’t even have sex — such as viruses.
The advantages of sexual intercourse
There seems to be no big explanation for sex, but there are lots of small advantages that together could be the reason why evolution decided to go with it. Here are a few of the main ones:
It provides robust genes. Sex can give rise to genes that are mostly ‘good’ and healthy; that are less likely to turn into ‘bad’ genes. These genes are also ‘isolationist’, meaning if they do turn bad, they are less likely to make their neighbouring genes also bad.
It has useful genetic drift. The constant reshuffling of genes with other partners does seem advantageous (even though asexual reproduction is also really efficient).
It can get rid of parasites. Some scientists believe sex evolved as a mechanism to escape vulnerability from parasites. Although this theory, known as the Red Queen hypothesis, has a lot of holes in it.
It may have been a response to death. Another theory is that sex came about after the evolution of death. About three billion years ago, the first complex cells began to die as they dumped toxic waste into the atmosphere. It is thought that by ‘learning’ to swap genes, these cells could obtain genes with new instructions to make room for DNA repair, and to deal with the toxic waste.
2. Different Sexual Desires in Males and Females
The reason why we have sex is a mystery, but the sexual conflict between male and female humans is well documented. Sexual conflict is very important to us, as countless love songs; stories, and poetry have documented. But what causes sexual conflict in the first place, and why can’t we all just get along? The answer is, because when it comes to sex, males and females often don’t see eye to eye.
What women want from sex
Before we talk about what it is women want from sex, it is important to note that before the introduction of the contraceptive pill and abortion rights from the 1960s onwards, women had a pretty big disadvantage when it came to sexual intercourse. Since the dawn of human evolution, women had to really make sure the man they wanted to sleep with was a reliable partner. It’s easy for a man to walk away from sexual intercourse, but for a woman suddenly pregnant, this could be the difference between life and death.
This type of thinking is hard-wired into the way women think as human beings. So even with all of the advantages of modern society, women still (like men) ‘think’ like they did thousands of years ago. This is because evolution is a slow process, and modern life is rapid. Modern life has rapidly outpaced our ‘Stone Age’ ways of thinking.
This is why women are very picky about the men they want to partner up in. Studies have shown that women want the following:
Good financial prospects. In Stone Age language, this translates to “having a lot of resources”. This makes it likely that her partner will be able to take good care of her and any children, whether this meant access to food or other privileges.
Characteristics linked with ‘social status’. This is similar to the above point. Women place great value on education in men — especially business and engineering degrees. But ‘social status’ can also mean a partner who has the respect of his friends, peers, and family, and isn’t a “pushover”.
A height advantage. Almost all women seek tall men, with 80 percent of women preferring men over six feet tall. This makes it harder for naturally tall women to find a mate, as their ‘pond’ is smaller to choose from. It is believed that the ‘tall’ quality is an indicator of greater protectiveness; able to command fear and respect in others.
Love. Almost all women require love in a relationship, even if their partner is seemingly perfect in every other way.
The importance of this last point, love, is the most strongly desired quality that women look for. Because sex is the most valuable reproductive resource woman can offer, they have evolved psychological mechanisms to resist simply ‘giving’ sex away. Requiring love, sincerity, and kindness — these are all ways a woman can feel confident that her partner is “the one” for her.
What men want from sex
Here are some of the most important qualities that men seek in women:
Youth. A young woman is associated with good health and fertility, hence why men all over the world prefer younger women — with the ideal age being 20.
Status. This is not the same as ‘social status’ (see above, for women). Men seek out women who have whatever features are linked with status in their particular culture. Perhaps the most obvious sign of status is physical beauty — the so-called “trophy” women that men strive for in society.
A curvy body type. Men typically prefer women with curves, although this is not what most women believe. The reason behind this dominant preference may be that, in maturity, more fat is stored in a woman’s hips and thighs (up to 40 percent more) due to the release of oestrogen. A curvy hip-to-waist ratio is a subconscious indicator of fertility and health to a man.
The most attractive. This means that even airbrushed pictures or ‘fake’ representations of beauty can result in a man being less attracted to ‘real’ women. This factor can put a big strain on modern relationships, not to mention the mental health of the women who strive to match unrealistic beauty standards.
Sexual loyalty. All over the world, sexual fidelity tops the list of men’s long-term mating preferences. In some non-Western cultures even virginity is a requirement.
This last point echoes the ancestral paranoia of mistakenly raising another man’s child as if it is one’s own. Women were often unfaithful if she felt her partner could no longer protect her. Sometimes infidelity would come about as the partner was off hunting for days on end.
Different attitudes towards casual sex
Unsurprisingly, males and females have totally different viewpoints when it comes to causal sex.Thinking about it from a biological point of view, it almost beggars belief why any man would want to settle down and fully commit — through marriage or otherwise — when it only takes a casual fling to impregnate a woman. There must be some evolutionary advantages for doing so. There are thought to be two main reasons for this: one is that, historically, children were far more likely to die without the prolonged investment from two parents. Another is that is is just so difficult for a man to have frequent casual sex, because woman won’t let them.
This study, by OKCupid, shows how men and women rate one another in terms of physical appearance. This data is specifically related to online dating, but the findings mirror with those in the ‘real’ world:
In terms of how men look at women, things seem fairly across the board. But for women, it is another story entirely:
As the above table shows, women appear to have unrealistic standards for the “average” member of the opposite sex. A similar study on Tinder revealed that 61.9 percent of males ‘liked’ (swiped right) on female profiles, but females only ‘liked’ a meager 4.5 percent of male profiles. In the OKCupid study, 0 percent of men scored “most attractive”.
This disconnect means that most men lose out when it comes to mating with women. In fact, many lie about the number of sexual partners they have; deliberately saying they’ve had more sex with women than they really have. Some men genuinely do sleep with hundreds, even thousands of women — but they are a tiny few. This is explained better by the pareto principle, a phenomenon where almost none of the men sleep with almost all of the women.
The “tragedy of the male sex drive”
Lust is what drives a man to have casual sex, and evolution dictates that a man should try to impregnate as many women as possible. The average man desires around 18 sexual partners in his lifetime, in comparison the average woman only desires about 4. A man’s inability to fill these sexual desires can severely impact his mental health.Take this quote from Roy Baumeister, a social psychologist:
The crux of the problem, for the average man, is that he wants sex with many women, but almost none of those women want him… He wants to have sex with many women but women don’t desire nearly as many partners, and so far fewer women will want to sleep with him than he desires… Men seem doomed to live out much of their adult lives in a chronic state of sexual frustration. They are doomed to be horny.
When it comes to casual sex, men are not bothered about a woman’s general attractiveness, her humour, intelligence, or even honesty. It might even be more appealing to the man if the woman is already in a relationship, because it minimises her desire to want to commit to the partner she is fornicating with.
Men are likely to find themselves more attracted to a woman in a nightclub or bar as it gets closer to the closing time. This “beer goggles” phenomenon occurs regardless of how much alcohol is consumed, and may be the male brain’s way of telling them to act on the decreasing opportunities for a late-night fling.
The Coolidge effect
There’s a legend that a US president, Calvin Coolidge, visited a commercial chicken farm in the 1920s, where his wife asked the farmer how he managed to produce so many fertile eggs with so few roosters. The farmer responded that his roosters were happy to do their duty dozens of times each day. The legend goes that first lady then said, “Perhaps you could mention that to the president”. President Coolidge then asked the farmer if the rooster services the same hen each time, to which the farmer replied: “Oh no, he always changes from one to the other”. To which the President is supposed to have replied, “Perhaps you could point that out to Mrs. Coolidge”.
Even if the story is not factual, the invigorating effect of a variety of sexual partners is now known as “the Coolidge effect”.
The Coolidge effect is a phenomenon that makes a man lose interest — or lessens his interest in — a mate he has already had sex with, and prefers instead to seek out a partner he has yet to sleep with. This phenomenon is observed in not just human males but male rats, rams, cattle, and sheep. This is why sex decreases in relationships over time, because a man will eventually desire a new woman over the one he already has.
Why does the Coolidge effect exist? One answer is the constant desire to seek out new genes from wide and varied partners, and as a guard against interbreeding and stagnating the gene pool.
Women and casual sex - a contradiction?
Casual sex seems at first to be a pointless exercise for women — because having multiple sexual partners at the same time does not increase fertility or make a difference to the development of a baby. It is thought that casual sex evolved in women as a mechanism to get resources in a time of need. Our ancestral female ancestors might have taken to casual sex in exchange for, for example, food during a drought.Here are some other reasons why women might have casual sex:
To evaluate a mate. Sex is a great way to get comfortable around a person. It also provides an opportunity for the woman to examine her partner and see if he is reliable, attractive, and trustworthy.
To have a back up plan. In our ancestral Stone Age days, deaths from disease or injury were common. Having a “go-to” mate meant a suddenly windowless woman wouldn’t have to incur the cost of searching for a new partner.
To fulfil the ‘good genes’ hypothesis. The thinking behind this theory is that women will seek out good genes to pass on to their children. In ancestral days, there was no sure way to determine who the father of a child was. So, even if a woman was in a relationship with a resource-committed man, it was evolutionarily advantageous for her to sleep with a better looking partner — if she could get away with it.
This last point may be men place so much importance on a woman’s sexual fidelity, and why men find it much harder to accept or forgive an adulterous partner than women. It is difficult to obtain the data, but it is estimated that as many as 10 percent of men could be unknowingly fathering a child that isn’t genetically theirs.
3. Sexual Conflict Between Males and Females
The differences in how males and females look at sex can cause distress, hurt, confusion, and anger in relationships.Here are some of the ways women use their evolved sexual strategies to ‘compete’ for men:
Name Calling. The scientific word for this is intrasexual female competition. Females denounce potential rivals by calling them fat, ugly, and by criticising their body shape and by generally doing anything they can to damage a rival’s self-image.
By playing “hard to get”. This is one way for a woman to test a man’s willingness to invest his resources in her, but it can also be a way to demonstrate to a man that she cannot be wooed easily by other males. On the flip side, men like it when women try to seduce them and act promiscuous, but if the woman is too overt this can be repulsive.
By testing a man’s investment. It is thought that women use moodiness and ‘nagging’ as a way to test a man’s commitment. If a partner is unwilling to tolerate her mood swings then his commitment is low, and vice versa. This is also a great technique to obtain valuable information about the sense of the bond in the relationship. But this is a fraught issue with men, who generally feel moodiness is a waste of time and resources.
By withholding sex. This is similar to the “hard to get” tactic, in which a woman has to tread a fine line between granting sexual intercourse too early (and being mistaken for a casual mate) and delaying it to the point that the male feels his time is being wasted.
And here are some of the ways men use their sexual strategies to compete for women:
‘Investment deception’. A third of all men have admitted to exaggerating the depth of their feelings in order to bed a woman, especially by saying “I love you”. But men can deceive economically too — by spending lots of money in the ‘honeymoon phase’ to give the appearance of excess generosity and wealth. On the other side, women often use ‘sexual deception’ methods — giving sexual hints in order to gain favours or services from men, with no intention of following through with them.
Pretending to be ‘just a friend’. Although men and women can just be friends, men find much more value with ‘friends with benefits’ than women. Men often mistake their female friends as moderately sexually attracted to them, even the women are completely oblivious. Once more, men are 62 percent more likely to break off a friendship is it becomes clear that sex is not on the table.
The fact that conflicts between males and females originate from our evolved mating psychology can be disturbing to some people. Our anatomy, physiology, psychology, and cultural traditions have all been shaped by the mating successes and failures of our evolutionary ancestors. No social relationship is untouched by mating. Fathers jealousy look after their daughters, making sure they choose the right mate, and daughters manipulate their fathers to mate with the men they love.
But sometimes, thankfully, we find lifelong love. And we can work together to improve and restore lost love, and overcome our sexual differences and conflicts. As individuals, mating influences much of what we do, and as a species, our mating rituals essentially define who we are.
4. Mysteries of human sexual intercourse
Af if sex wasn’t already complicated and mysterious enough, there’s a whole library full of journals, research papers, and books on the unexplained aspects of human sexual intercourse alone. Here are two of the biggest ones that you might have heard of…
There have been many theories floated over the decades to explain homosexuality, and none of them are convincing. What is understood though, is that homosexuality is heritable. This means that the traits for homosexuality are prescribed by genes that are different from those that lead to heterosexuality.
It is also understood that hereditary-influenced homosexuality occurs too frequently around to world to be due to ‘accidents’ in the DNA. So, even though homosexuality stops people from having children, it must be favoured by evolution for some other reason.
One of the most popular theories is known as the kin-selection theory. It’s the idea that homosexuals may have evolved to help rear and look after children. Historically most men failed to reproduce with a woman. In this instance, it may have been evolutionarily beneficial for some men to spend their time and resources investing in family instead. The problem with this theory is that it doesn’t explain why homosexual men wouldn’t just be born asexual.
The female orgasm
The puzzling thing about the female orgasm is that — unlike the male orgasm — it isn’t a necessary part of conception. One scientist, Stephen J. Gould, believes the female orgasm is a “functionless by-product” — something that exists but doesn’t really serve a purpose, like nipples on a baby boy.
Approximately 15 percent of women always experience an orgasm during intercourse and seven percent never do, and women find sex immensely enjoyable whether they orgasm or not. There are even some human cultures on Earth where the female orgasm is nonexistent!
Here are some proposed explanations for the existence of the female orgasm:
To encourage sexual intercourse. Women who orgasm more are thought to be more likely to want to have more sex, and with more partners — a practise that will increase the odds of being impregnated.
As a way to signal sexual loyalty. A woman who orgasms is presumed to be selecting a man who will stick around and invest in her and any future offspring. It may also be a way of telling her partner that she is enjoying sex and is not looking to seek sex from another.
As a way of sperm retention. Some studies have suggested that the female orgasm essentially traps sperm, increasing the chances of pregnancy.
As a measure of trust. According to the Mr. Right theory, most women only orgasm during intercourse with a partner they know. Almost 77 percent of all one night stands never involve a woman’s orgasming. Married women are more likely to experience orgasm than their none-married peers.
There are plenty of theories, but nothing concrete to explain the existence of the female orgasm, but evolution seems to have carefully adapted it to happen when and with whom it occurs. So if these clues are anything to go on, perhaps it won’t be a mystery for much longer.
5. Secrets of a successful marriage
We are driven by our instincts, but we are not bound by them. Our genes might command us to reproduce all the time, but we don’t listen to them by using contraceptives and birth control.
Understanding the sexual evolutionary mechanisms that shape us is liberating, because it better allows us to understand our in-built weaknesses, desires, and subconscious goals. And only by knowing these factors can we plan around them, and make a better life for ourselves, our partners, and our children. One free of sexual conflict and negativity.
The implications of human mating psychology for happy, lasting marriages might seem intense, but they are not. Marriage is a commitment and we are only human. To preserve a marriage, couples need to be faithful to one another; have children together; have ample economic resources (and this doesn’t mean ‘lots of money’) — but, most importantly, must be kind, trust, love, understand, and be generous to one another. With all of these factors in place, there’s no reason why a marriage shouldn’t be fruitful, filled with love, and last forever.
Brooks, M. (2010) 13 Things That Don’t Make Sense: The Most Intriguing Mysteries Of Our Times. London, UK: Profile Books Limited
Buss, D. (2003) The Evolution Of Desire: Strategies Of Human Mating. New York: Basic Books (Perseus Books Group)
Neyt, B.; Baert, S. (2019) Are Men Intimidated By Highly Educated Women? Undercover On Tinder. https://www.sciencedirect.com/science/article/abs/pii/S0272775719301104
Baumeister, R; Tice, D. (2000) The Social Dimension of Sex. Allyn & Bacon.
Ryan, C; Jethå, C. (2010) Sex At Dawn: How We Mate, Why We Stray, and What It Means For Modern Relationships. New York: HarperCollins publishers
Wilsom, E. (2012) The Social Conquest Of Earth. New York, USA: Liveright Publishing Corporation