I recently came across a rather strange question.

If you were given the chance to be a person of the opposite sex for two days every week, would you take it?

So I'm going to take this question one step further. What if this trait was genetic and unavoidable? Every weekend, every person in the world becomes a person of the opposite sex.

Specifically, how would relationships work? I see two possibilities:

  • People could be polygamous. Every weekend, they would have a different partner to their usual week time partner.
  • People could stay with one partner as they would also change sex - but how would they recognise each other the first few times?
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    $\begingroup$ What would happen if you were pregnant? $\endgroup$ Commented Jan 9, 2015 at 9:54
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    $\begingroup$ The classic SF treatment of this question is "The Left Hand Of Darkness". Also, there are a number of this-world couples where one partner has changed gender(expression) but the couple have remained monogamous. $\endgroup$
    – pjc50
    Commented Jan 9, 2015 at 12:16
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    $\begingroup$ Does only the sex change but also the gender and sexual orientations? Yes, these are different concepts. $\endgroup$
    – Philipp
    Commented Jan 9, 2015 at 13:05
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    $\begingroup$ People are already largely polygamous even without gender change. $\endgroup$
    – user4239
    Commented Jan 9, 2015 at 17:02
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    $\begingroup$ @KSmarts Either. Both polygamy and polyamory count here $\endgroup$
    – ArtOfCode
    Commented Jan 9, 2015 at 18:54

3 Answers 3


I think you might be approaching this idea too much from a western, heterosexual mindset.

If this were something that has always existed in a society, two things can be said: one, it probably arose for an evolutionary reason, and two, it predated the establishment of culture.

There are many species in which gender works differently. I am far from a biologist, but I am aware many aquatic life forms and insects either have unusual versions of the common male/female, are hermaphrodites, or can change gender on command. The main reason, to my understanding, that this does not occur in larger life forms is that they posses larger, more complex reproductive systems, making changes more difficult, and possessing both sexual traits more costly to physical resources.

However, if a life form was developed with uncontrollable gender changes, its culture would develop around that. Within anthropology, it is often said that gender is a social construct. Many societies possess third genders or allow switching between them. Obviously, this is a cultural construct, and does not involve physical change in sex (although these days, it can mean that). While it is more politically contentious, based on that I would say it stands to reason sexuality is also a social construct.

What that means for your question is that the idea of being a given gender in the first place would likely never arise if sex was not fixed, or if it did exist, would be based on some other phenomena (perhaps choice). Someone might identify as "female" despite actually being male half of the time. Similarly, it seems probable everyone would end up bisexual, due to physical necessity. As such, there would be no particular reason to believe marriage would work any differently on that premise alone.

However, it may indeed impact the idea of marriage. It could lead to the idea never developing, for one. It could also lead to three-party marriages, or some other system. It is almost impossible for me to predict the impact it would have on marriage, though, because marriage is already defined very differently in some parts of the world. Throwing something as large as changing genders into the mix would confuse the situation beyond what I think a single, clear answer exists for.

Of course, I just assumed this was always the case. It is also possible that those possessing this gender changing effect were created out of an already existing species, like humans. In that case, I would suspect that at least for some period of time, our cultural constructs would persist - polygamy or simply abolishing of marriage seems the more likely short-term result, as sexuality and gender are more deeply rooted in the cultural mindset.

To address the matter of changing how people look; really, there is no reason to think people would look outwardly any different, let alone different enough it would be an issue. Especially if you're posing this as a genetic phenomena, there would not be a major physical change over that short a time, if there was even any at all. Perhaps if it was magical, and there was a total change in body structure, but even then I can't imagine it being more confusing than someone dying their hair. There would still be the same dress, mannerisms, etc..

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    $\begingroup$ Excellent answer! Retracing the biology from the dawn of the species is really the best way to suss out how the surrounding culture would evolve. $\endgroup$
    – Lindsey D
    Commented Jan 9, 2015 at 17:15
  • $\begingroup$ Based on the description in the OP, such a species could very well wind up defining gender based on when a person is male or female, rather than on which the person is "normally" (though 5 days one 2 days the other does give one a bigger timeshare than the other). $\endgroup$
    – Brilliand
    Commented Jan 9, 2015 at 19:06

Just changing sex wouldn't drastically change your appearance, you would easily recognize each other (and could always tell each other who you were if not).

This question is impossible to answer because there would be no single solution. Some people would pair up and just abstain for two days, some people would enjoy the variety of experiences as a couple, and other people would be polyamorous or swingers...just like in our world.

People are people, I really don't see this making any big changes in terms of relationships - although it would most likely reduce sexism and homophobia.

Of course there is one big problem...How would pregnancy work in this concept?

  • $\begingroup$ Pregnancy would have to work like the change itself does, ie the fetus would simply count as part of the womb until birth and change accordingly. Being as a change this fast would have to be magical anyway, that is not really an issue. $\endgroup$ Commented Jan 9, 2015 at 10:52
  • $\begingroup$ @VilleNiemi I think what he means is, how would the fetus develop? What would happen to it when its mother suddenly becomes male and no longer has a womb? $\endgroup$
    – KSmarts
    Commented Jan 9, 2015 at 16:45

If every person's change occurs at the exact same time (every weekend), then I'd argue that polygamy would be no more or less likely than in our current world. If John and Mary are a couple, then the weekend happens and John becomes "female" and Mary becomes "male" at the same time, then from a biological perspective, they're still a fit couple.

This, of course, raises an issue of sexuality: If John is attracted exclusively to women, and this weekend change occurs, is he still willing to have sex with Mary, who is now a male and therefore unattractive to him? Obviously, the couple can just abstain as someone previously mentioned. I'd be willing to argue, however, that although John isn't attracted to Mary's physical form, he would be attracted to his own body and have a natural interest in pleasuring it the same way he would be interested in pleasuring another woman. And what's the best way to do that? Get a man like Mary involved.

If (as is far more likely) each person's change occurred at different times (Mary is a man Monday and Tuesday while John is a woman Friday and Saturday), then I believe that polygamy would be much more likely. Again assuming that John is exclusively attracted to women, on Monday, when both John and Mary are men, John can derive little pleasure from any sexual activity with Mary and therefore would most likely seek it elsewhere.

In this world, where changes aren't simultaneous, I would predict very few couples, with most people opting to include at least a third person. Two-person couples would probably be limited to those who consider themselves bi- or asexual, or those for whom their "schedules" line up. Alternatively, couples are as likely to be polygamous as in our world and everyone is bisexual.

To address the pregnancy issue: If this is a biological process, then we have to assume that this hypothetical species has been able to reproduce (which would be impossible if the fetus were aborted every time the change to male occurred). As that is the case, these particular humans would have to have most of the inner workings of a female even in both gender forms, the same way that both genders have a stomach or eyes. It would be optimum to have the "male" (the person who is male 5 days out of the week) impregnate the "female" still, since that means that the baby is far more likely to enjoy a natural birth instead of having to be born surgically. That being the case, I would assume pregnancy would still be something associated with the "female" gender.

What that means for your question is that the idea of being a given gender in the first place would likely never arise if sex was not fixed

The problem with this is that although gender isn't fixed, a person is one gender five out of seven days of the week. So a person is still one more often than the other. I think that your predominant form would determine your cultural gender.


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