The Purpose of Sex in Humans?

It’s easy to believe the natural purpose of sex is reproduction.   Offspring, after all, are the single most spectacular result of sex.  Many of us seem bedazzled by that fact.  Consequently, some of us seem to have got the notion the natural purpose of sex is reproduction and that sex without the possibility of reproduction is at best selfish indulgence and at worse perversion.

A Muslim friend tells me Islam forbids birth control on the grounds that sex is divinely ordained for reproduction.  I have heard much the same thing from Catholic friends, and from fundamentalist Protestants.

Even though I think my friends are wrong (and they think I am wrong), I am not going to argue with them in this post because I am concerned here only with the notion that nature has ordained a purpose for sex. The notion some god has ordained a purpose for sex is in some crucial ways quite different from the notion nature has ordained a purpose for sex.

Maybe the best way to approach the question of whether nature has ordained a purpose for sex is to ask what human sex would look like if it’s primary purpose were indeed reproduction.

If that were the case, then I think we might expect human sex to more closely resemble sex in most other mammals.  Mammalian females — including human females — are not always fertile.   In fact, females are more likely to be infertile than they are to be fertile.  Given that fact, if the natural purpose of sex was reproduction, then it would not make sense to have sex when the female was infertile.  It would instead make sense to have sex only when the female was fertile.  And in most mammals, that is the case — sex occurs almost exclusively when the female is in estrus or heat.  That’s the phase during her reproductive cycle when she is most likely to be fertile.

Yet, humans are not like most other mammals.  Instead of mating only when the female is in estrus or heat, humans mate both in and out of estrus.  That one fact, when properly understood, by itself lays to rest the notion that reproduction is the primary natural purpose of sex in humans.  For why would human evolution discard the arrangement of mating only when the female was most likely to be fertile, if the primary natural purpose of human sex was reproduction?

Even though that one fact is all that’s needed here, there are several other facts that also make nonsense of the notion the primary purpose of human sex is reproduction.   Among them in this: Estrus in most mammals is highly marked.  That is, females clearly signal they are fertile.  But in humans, estrus is almost indistinguishable from other, non-fertile, phases of the reproductive cycle.  Why would human females no longer signal when they were fertile if reproduction were the primary purpose of human sex?

Again, post-pubic human females are sexy even when they are not fertile.  It seems that in most mammals, the females undergo changes that make them sexually attractive — or sexy — only during estrus.  However, a case can be made that human females are more or less constantly sexy.  For instance, a female chimp’s rump swells up during estrus.  On the other hand,  a human female’s rump swells up at puberty and remains constantly swollen thereafter, regardless of where she is in her reproductive cycle.  But why would human females be more or less constantly sexy if the primary purpose of sex were reproduction?

Those are three factual reasons for rejecting the notion that nature has ordained reproduction as the primary purpose of human sex.  But there is also a philosophical reason to reject the notion.

The argument that nature has ordained reproduction as the primary purpose of sex rests on the assumption that nature has purposes.  Yet how can nature have purposes?  In order to have a purpose, you must have a will, and nature has no will.   So, instead of speaking of the purpose of sex, one might speak of the function of sex.

It seems pretty obvious that one of the functions of sex in humans is reproduction.  But it is equally obvious that another function of sex in humans is bonding.  And there might be other functions that are less obvious.  But whatever functions there are to human sex,  we should be very clear there are at least three factual reasons and one philosophical reason to refrain from stating that the primary purpose of human sex is reproduction.

25 thoughts on “The Purpose of Sex in Humans?

  1. The idea that sex in humans — or hominids, for that matter — is [i]only[/i] for procreation is an opinion that demands the holder ignore important data and evidence, such as the human need for warmth, stroking (daresay, “grooming”?), touch, and the emotional component of sex. In fact, I’d say the idea of “sex as only procreation” is a model which probably originates in a distinctly and decidedly male view of sex, one which denies the need for anything other than a physical function to sex.

    So, if a couple are medically infertile, and procreation isn’t possible, does this mean they should be prohibited from “indulging” in sex? Seems that would be consistent.

    I quibble with the post in that I think there’s [b][i]plenty[/i][/b] of evidence that other mammals engage in sexual activity quite apart from an estrus cycle. (See http://scienceblogs.com/zooillogix/2008/07/animal_sex_its_not_just_for_re.php.) No doubt more would, if the female didn’t take the male’s advance as an encroachment and turn it into a wrestling match!

    There are many other links of interest, most from biologist Professor PZ Myers:

    http://scienceblogs.com/pharyngula/2008/11/god_and_sex_two_potent_ideas_t.php

    http://scienceblogs.com/pharyngula/2006/06/evolution_and_homosexuality.php

    http://scienceblogs.com/pharyngula/2009/07/i_get_email_followups.php (the comments are the better part of this)

    http://scienceblogs.com/pharyngula/2006/02/tentacle_sex_part_deux.php

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  2. A very well conducted reasoning. Although one often has the impression our reason is inadequate to ‘make sense’ of things and especially of ourselves (biology, man etc.: while for example mind seems more successful with physics).

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  3. Sex with feelings encourages bonding but without any feelings is nothing but lust satisfaction. at times sex is one of the ways to let out your frusrations,your anxieties . i read somewhere that sex helps in reliving tensions and at times tiredness.
    so each one to his own reasons but the fact remains it is an integral part of life

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  4. Sex for procreation only is a survival concept. Sex, as has been said is a way of bonding and demonstrating one’s love for the partner. If done solely for the physical satisfaction it can be frustrating. Solitary masturbation is a case in point; you tire of it and look for something more stimulating as any teenager will tell you.
    If sex is only for procreation then it is forbidden for older people past their fertile years. Granted it happens later in men but women, after 45 or 50 would be out of the loop and would not havre any desire for it…which of course is not the case. The wife and I will vouch for it anytime.

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  5. Interesting post Paul.

    There have been various theories put forward in explanation of what you discuss, the technical term being “concealed oestrus”.
    I favour the theory that states that fertility is concealed in human females so that she may select the best quality mate for reproduction, whilst maintaining a relationship with a genetically inferior male that nevertheless may have other desired qualities (e.g. good father, influential in the community).
    Having her cycle concealed from her primary mate allows her to produce offspring with a genetically superior male whilst evading any social repercussions that may arise with her long-term mate.

    Imagine a scenario where a tribal chief has many wives, he has the greatest power and influence, which will prove beneficial to the raising of children, but genetically there may be better mates available to the women.

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  6. Hi Paul,
    There are few things today that we can truly enjoy doing. Its also uncertain how long before some meteorite or a random nuclear bomb wipes us off. Newspapers are full of such stories. One can either choose to believe in some random rules that someone claims someone else made, or do what their heart says is right.
    If I had another life cycle left, I’d hope to be reborn as a dolphin. They are the only other mammals who are known to have sex for recreation. 😉
    Priyank.

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  7. Final_Transit said; “If I had another life cycle left, I’d hope to be reborn as a dolphin. They are the only other mammals who are known to have sex for recreation.”

    Heh, did you know that sex is used by bonobos to say “hello”?

    At uni I knew some people with similar social habits…

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  8. I’m reading Matt Ridley’s The Red Queen. He suggests that sex evolved as a means of strengthening resistance to parasites. Asexual reproduction does not provide genetic variety; offspring are essentially clones of their mothers. Sexual reproduction mixes up the offsprings’ genes, thereby creating new, unique genetic combinations that parasites have more difficulty subduing.

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  9. @[i]the chaplain[/i],

    I believe it was the statistician R.A.Fisher who once said sexual reproduction was a process for creating the highly improbable. This characteristic, as the strengthening of resistence to parasites hypothesis, is far from unique to humans. Invertebrates even exhibit it.

    In fact, sexual reproduction isn’t a surprise at all, since it is a means of creating highly diversified genomes, along the lines of Fisher’s suggestion. Plants do it, too. This is what you’d want if “survival”, as evolution defines it, means having as many viable offspring as possible, where viable means fertile and surviving until reproduction.

    That said, we sexual beings ought to realize there isn’t anything particularly special about it. Unicellular organisms have developed many ways of exchanging nuclear material. Some of these methods of “horizontal genetic exchange”, are quite clever. See http://www.journals.uchicago.edu/doi/pdf/10.1086/514917

    Sexual reproduction is something some multicellular organism picked up about two billion years ago, and we’ve been stuck with it since!

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  10. I think they answered this question in slate a few years ago. According to their research, humans are the only mammals to have sex all year due to social needs and the need to keep the father of children around. 🙂

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  11. Sex for procreation only (and within the confines of marriage) seems like a last-ditch effort of religious conservatives (pick your religion; you’ll find conservatives).

    When you look at cultures that have historically (and continue to) mutilated female genitalia (as so-called right of passage), you could also consider the aspects of sex as male mechanism for power, control, etc. A whole other discussion.

    It remains interesting to me how much of a role culture plays in all this – familial and societal. Having lived on both sides of the pond at various points in my life, the differences in treatment of sexuality are significant; I’ve experienced European cultures (and families) where it is considered natural – in all its forms and varied purposes, and I’ve experienced both American and European cultures (and families) where (usually) a more narrow range of allowances for sexual activity are tied, in some way, to religious beliefs.

    An endless interesting set of subjects.

    Speaking of which – I’m curious to know what you think of my handling of the sexual fantasy topic (certainly not scholarly, though I did a fair amount of research).

    I don’t think the existence and importance of sexual fantasy would lead one to believe that sex is for procreation only. It certainly leads us to enjoy, and simultaneously shows why we are legitimately “the human animal” – and yet one more way in which we differ from the other animals.

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  12. Pingback: "Pro-Gay Activist Admits It: Bullying Hysteria May Cause Suicides, Not Prevent Them" - Page 2

  13. Women really don’t like to have sex outside their fertile time (when is best), the big difference from other mammals is that all women have at different times, the cycle is individual rather than sinchronized, of course this have the males sort of confused, because although you may not be pheromon by your mate, you are by other womet, keeping men in a state of readiness to mate . does this make more sense to sticking to the purpose of sex and think of sex acts as different (simulated) sex?

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  14. Human sex like so many other human activities has evolved and diversified to meet the needs and desires of a wide diversity of humans on this planet. Another modern motivator for sex (with no value judgement on my part) can best be described as entertainment. This is similar to (but not exactly the same as) the evolution of running, throwing and other physical activities into modern sports, or voicing into acting and singing. Sex which clearly originated (before our species) as a means to procreate the species has evolved over the eons to be an activity that meets a wider diversity of desires and needs than the narrow functionality it originally served.

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  15. Let me weight in this topic. I think you misinterpreted your friends honestly. Procreation/Bonding is the PRIMARY purpose of sex is what believers believe. If you were the first humans, your ultimate goal would be to make sure your seed doesn’t die and your goal would be to make your population bigger. You have this view because of the environment around you. Let’s take car for example. The primary purpose of car is to make movement more faster and more free. But why do people buy the latest model. Reason is because it comes in a more presentable form, with new gadgets and also the pleasure and also the need. But the car is JUST TO TAKE you places. And always laugh when people justify something by using animals. Animals are different from humans. We might be classified as such but we know there is a difference between the two. Like animals don’t stay with their partners. Also purposes is dependent on the actors. It depends on the people taking such action. Both you and friends are right. The bible does mention other purposes of sex like intimacy, bonding, read book of Solomon. The only difference is that sex is within the context of marriage. The topic is as social and also scientific. After the sex revolution, the amount of sex outside marriage increased. From the tone, you open an arena of illogical bashing and continue the religion v science bullshit.

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  16. to ayoiawe: animals are used all the time to study human behaviors. if what you say is right, then no one would be able to use lab mice to draw an analogy between their behavior and ours.

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