In pillbugs, sex is determined by two chromosomes: Z and W. Individuals who inherit two Zs develop as males, while ZW individuals become female. But in some populations, these rules are overwritten by a microbe called Wolbachia.

Wolbachia infects the cells of pillbugs and only passes down the female line; only mothers can transmit the bacterium to their young. Male embryos are dead ends to Wolbachia, so when it runs into them, it feminizes them by interfering with the development of hormone-producing glands. The result is that all young pillbugs infected with Wolbachia grow up into females, even those that are genetically male. In such populations, the W chromosome tends to disappear altogether. Eventually, all the pillbugs are ZZ, and it’s the presence or absence of Wolbachia that dictates whether they become female or male. . . . In the 1980s, the French researchers showed that some pillbugs do not have Wolbachia, but act as if they did. They’re all ZZ, but some still develop as females. The researchers proposed that the bacterium has transferred a piece of its DNA into the pillbug’s genome, and that this “feminizing element”—or f-element—was now dictating the animal’s sexes, even in the microbe’s absence.

From The Atlantic Magazine.

What if a bacterium, with a similar effect on humans to Wolbachia in pillbugs (i.e. initially turning males into females until Y-chromosomes disappear and then eventually adding its own sex determination gene), rapidly spread through our world in the early 21st century?

This occurs because the bacterium infects the leading, internationally distributed bottled water brand, staring in the year 2014 and infects 80% of all people who every drink and bottled water in the time period from the year 2014 to the year 2029.

The source of the bacterium is not discovered until fifteen years after the bacterium had initially started to spread, because of a clerical error in processing the data that addresses possible causes early on, that is published in a prestigious scientific journal. This error is not be caught by anyone early on. The data published in the journal appears to rule out the bacterium that is responsible as a cause, sending scientists looking for the cause on a wild goose chase for another cause.

Assume that the bacterium is an otherwise symbiotically helpful gut bacterium.

How would the world react and change as a result?

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    $\begingroup$ I changed hard-science to science-based because «how does the world react» is not suitable, and existing good answers are not hard-science. $\endgroup$ – JDługosz Dec 16 '16 at 12:13
  • $\begingroup$ "How would the world react and change to X?" is a very big question. This includes changes to society, culture, religion, economics, athletics... Voting to close as too-broad. $\endgroup$ – Frostfyre Dec 16 '16 at 13:22
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    $\begingroup$ This edit is being discussed on meta. Some of these comments are not very nice; people are obviously unhappy, so let's try not to make it worse, ok? Please take the discussion to meta. I plan to delete these comments soon. $\endgroup$ – Monica Cellio Dec 16 '16 at 14:55
  • $\begingroup$ Both parties have now posted on meta regarding revisions to this question, so I've cleaned up the commend thread. Any future points should be brought to the meta post; further comments will be deleted. $\endgroup$ – HDE 226868 Dec 17 '16 at 2:25

I think that it's fair to say that the effect on the world would be much less marked than if the opposite happened (significant over-population of males) - as it shouldn't affect the birth rate anywhere near as much. Men can clearly father many more children than women can, and can go on siring offspring for a long time. Socially it is possible that polygamy would become far more acceptable, especially in societies where it is felt necessary to keep the birthrate up. Age-difference in relationships would also become more significant.

Past that, it would affect the rest of society to some extent; assuming that the issue only lasts for 15 years and basically goes away when it's discovered then the imbalance isn't that great. Whilst your figures would give you 90% females for that period, with its only being around 20% of the average western lifespan you'd only end up with a 42:58% split, which is significant but perhaps not completely world-changing. For example, if you look at any field that's male-dominated and then add 1/3 to the number of women in that field, then you'll probably still find it comes out male-dominated. For example, the UK has 650 members of parliament, fewer than 200 are female (so more than 450 are male). Change that to 260 female MPs, and it's still 390:260 male dominated.

  • $\begingroup$ Interesting calculations. $\endgroup$ – ohwilleke Dec 16 '16 at 21:56

I won't discuss the actual biology, which is of course much different in mammals and feathered dinosaurs; let's take it as granted.

The premise is intriguing and yet quite common: what would happen if the sex ratio changed dramatically. For some reason, the sex ratio always seems to change so that there are many more women than men... The obvious consequence would be a significant increase in the amount of beauty in the world; it is also quite likely that there would be important consequences in the customs of those countries where most children are born by married women.

Practically speaking, any moderately large imbalance in the sex ratio would be detected quickly and a lot of money and effort will be expended to find out how and why it happens. The usual epidemiological methods will be applied to determine the place of origin and the means of spreading the affection; the identification of the pathogen may of course take longer.

Since the means of transmission of the pathogen seems to be important, I feel that the consequences of chosing to spread the pathogen through an "internationally distributed bottled water brand" must be explored. Let's ignore the problem of getting the pathogen into the bottled water produced by a reputable bottling company and suppose that such a gross negligence can go undetected for some time. (Maybe the pathogen does not get in the water accidentally but it is the work of a villain or of a determined idealist.)

The brand itself is an abstract concept and cannot carry any pathogens. Excluding places like Dubai or Qatar, as a general rule, the vast majority of people drink water obtained from sources not very far away; the popular bottled water brands are almost always local, because they are much cheaper; in the case of bottled water the cost of production is dwarfed by the cost of transportation over long distances. (One of the first tasks when living for more than a few days in a new country is to find which of the local brands of water has a sufficiently agreeable taste.) Those brands which are moderately popular in multiple countries, for example PepsiCo's Aquafina, are just brands: the actual water is sourced and bottled locally. Only luxury brands such as, I don't know, maybe Evian, transport actual water over long distances.

What this means that if the pathogen is distributed worldwide by bottled water the brand is a luxury brand; and only (very) rich people drink such brands of water. The immediate consequence is that in most places the sex ratio imbalance will be limited to the fabled one percent who rule us all; and, curiously, while in most countries only very rich families find themselves producing only daughters, in parched countries this affection is shared by a larger proportion of the population. This may give an early clue to the epidemiologists hunting the pathogen.

If the villain can persist in infecting internationally distributed luxury water for enough time, and if the various national health agencies manage to miss a pathogen distributed by bottled water (which, after all, is supposed to be microbiologically pure) then we'll see a lot of very rich women; the top financiers, the top captains of industry, the most powerful men in the world will be women.

I can't wait to see a meeting of the European Council where almost all of the participants are female...

P.S. If I'm not mistaken the question is about sex (the biological category) and not gender (the social category). There could indeed be a bacterium which interferes with how humans perceive their social gender, but I think that this is not the focus of the question.

  • $\begingroup$ You bring up an interesting plot idea: the selectivness suggests a target. And intentional attacking explains why it’s not detected with normal procedures. $\endgroup$ – JDługosz Dec 16 '16 at 12:48
  • $\begingroup$ Is there a sex tag? I was aware of the distinction but found gender and assumed (perhaps wrongly) that there wasn't a gender tag. I haven't resolved whether the cause is intentional or accidental and may deliberately decide to refrain from doing so. But, the notion of bottled water distribution was to create different levels of incidence in different populations as this post explores. $\endgroup$ – ohwilleke Dec 16 '16 at 21:52

The reaction would overall be minimal. Even now we have gender equality. So, having a male or a female leader is not an issue. They are treated the same.

What would be interesting is the reaction in male dominated societies such as in many Asian countries. In Saudi Arabia, AFAIK, women are not allowed to drive without the accompany of a male mukhrim. Mukhrim is someone who is blood related. With the outbreak there might be families without any male. The law might be repealed.

The overwhelming number of female might also lead to harem. Polygamy might be rampant is countries with strong male-dominated culture. Because it is in human nature to mate, males would be hot comodity. Even the nerds and otaku would have a better chance to score with girls.

Manufacturing companies would shift more to products that appeal to women. We will see more bags and dress ads and less suit ads. The next James Bond would still be a male since it will sell better due to the audience being mostly females. But we wil see an increase in soap operas. There would be more BL anime and manga than harem ones.

But overall the change wouldn't be drastic.


If it took 15 years to find out the cause of a widespread society-damaging disease, I can only assume that the society has quickly collapsed. I'd expect the research to take a year or two, at most.

A single publication disproving bacterial nature of the disease won't stop the competing labs from double-checking the results. Different labs from all over the world will be looking for the cause, simultaneously and, mostly, independently, and a gender-swapping bacteria will likely be the main hypothesis. It sounds really, really implausible that no lab manages to detect this bacteria - the Wolbachia have been identified in 1924, so I guess the modern technology is pretty good at finding bacteria.

If there was no collapse (they still seem to have bottled water in 2029), then once the bacteria is found, people will develop some antibiotics or bacteriophages, kill it, and get back to mostly normal. So you'll get ~20 years demographic anomaly; it'll cause some cultural changes, but no real apocalypse.

  • $\begingroup$ I wouldn't think it would cause an apocalypse, but I would be surprised if a 20 year demographic anomaly didn't have profound changes in our society and if the end result was a change in germline DNA that created two independent forms of sex determination, one in infected people and another in uninfected populations, as in the pillbug case, a cure wouldn't end the lasting effect. $\endgroup$ – ohwilleke Dec 16 '16 at 21:55

Something similar has happened due to another well known affliction: War

Historically, only men would be sent to war, meaning that heavy casualties would result in an imbalance in the population.

One example was the "Guerra del Chaco" in which Paraguay, with an approximate population of over 1 Million, lost around 150000 men.

From a social standpoint, the result wasn't too dramatic, as social norms stood strong despite the imbalance, but there was a boom of "women-focused industries", so to speak, like modelling.

Or at least that's what I was told when living there many years back, I haven't been able to find more concrete information.

So back on topic, a drastic imbalance in gender dynamics would probably only result in societal changes if the social norms are flexible enough to allow change. For example, the effect would probably be completely different in a country like say, Norway, than in a country like Saudi Arabia.

  • $\begingroup$ Another example of gross gender imbalance was the post-Civil War South. And, it would very much doubt that it didn't affect social norms at least somewhat. The question is really which ones and how much. Also a 15 year shock of that magnitude would be pretty much unprecedented. There have been very deadly wars and very long wars, but no wars that have been both that deadly and that long that I am aware of. $\endgroup$ – ohwilleke Jul 17 '18 at 17:25

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