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Research shows that some creatures in the animal kingdom manipulate the sex of their child in order to maximize the number of grandchildren. Parents in good condition, based on health, size, dominance or other traits, would invest more in producing sons, whose inherited strength and bulk could help them better compete in the mating market and give them greater opportunities to produce more offspring. Conversely, mothers in poor condition would likely play it safe, producing more daughters, whose productivity is physiologically limited.

This setting takes place in the ancient world, before advances in medicine and technology. I want to biologically alter humans in a way that allows individuals to manipulate the sex of their children. How could this work? Would it make sense to put this ability in males or females?

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    $\begingroup$ My eyebrow has just risen to the sky... It takes time for a mother to give birth to a child. During this time the mother cannot make other children, while the father still can. If you wish to maximize the number of grandchildren you want a very skewed gender representation, with very few males and a lot of females. Productivity is no issue, as very (very very) few animals that we know of reached a natural resource bottleneck. $\endgroup$ – NofP Jan 16 at 15:15
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    $\begingroup$ It makes no sense to "invest more" in sons if the primary goal is lots of grandchildren. Men can produce a thousand children in a year (okay, a couple hundred is more realistic, if the culture allows) and women only 1-3, and they need breaks. So you want more women than men or the men run out of non-pregnant partners. Nor does the mothers "playing it safe" make any sense at all. I recommend a pretty heavy rewrite. Your questions are usually a lot more thought-out. $\endgroup$ – Cyn Jan 16 at 19:37
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    $\begingroup$ @Cyn I don't think you understand the question. Think about the question not as a species working together to grow as large as possible, but from the true evolutionary perspective of individuals in a species competing against each other to produce the most offspring. In a species where males compete for reproductive success producing weak males that can't compete is suboptimal. You'd be better off having a female whose fitness is less dependent on their strength. On the other hand, producing strong males that will outcompete their peers will have greater expected fitness than having a female. $\endgroup$ – Mike Nichols Jan 16 at 19:52
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    $\begingroup$ @MikeNichols this is the ancient world, with societies and not a "Grok best hunter and therefore mate" world. Nor would skewing the balance between males and females make the males less likely to be "strong" however you define it. if parents raise sons in a resource-heavy way, fewer sons and more daughters is an advantage there because a higher percentage of sons will be "strong." Also, if you expect a woman to produce children almost nonstop, she will have to be very physically strong (and even then, it's not easy). More daughters lessens that burden. $\endgroup$ – Cyn Jan 16 at 20:07
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    $\begingroup$ @MikeNichols I still don't buy it but you should write it up as an answer (with some genetic alternation or whatever is needed). While it's true that an individual couple might have more grandchildren if they have more boys than girls, it only works for a population if there are enough girls. Otherwise, it's a pyramid scheme. This is not something that would evolve (your link shows that) & genetically engineering it wouldn't do much after a couple generations. $\endgroup$ – Cyn Jan 16 at 21:12
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It's very simple, infanticide. Death is a natural process and infanticide has been practised and probably still is in some societies. I shall try to avoid getting entangled in the abortion controversy. The ancient infanticide solution comes after birth of the child when the gender is visible.

Infanticide, the killing of unwanted babies, was common throughout the Roman Empire and other parts of the ancient world, according to a new study. http://www.nbcnews.com/id/42919668/ns/technology_and_science-science/t/archaeologists-document-ancient-baby-killing/#.XD8uUFz7Tb0


China

Marco Polo wrote about seeing many babies exposed in Mani [11]. Sex selective infanticide was common in China . Han Fei Tzu, a Chinese philosopher in the 3rd century BC wrote that “As to children, a father and mother when they produce a boy congratulate one another, but when they produce a girl they put it to death”[12] . Many Chinese tribes practiced infanticide by putting the baby into a bucked of cold water, called “baby water” [13] . The 2006 edition of a book by two prominent historians stated that even by now, in China, infanticide seemed more likely for a girl than a boy.

https://www.wikigender.org/wiki/history-of-infanticide/

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  • $\begingroup$ This is the correct answer, the only answer available to the human species in "the ancient world" $\endgroup$ – Pelinore Jan 16 at 16:17
  • $\begingroup$ There is an even easier form that may actually happen spontaneous abortion, it requires a lot less energy than carrying a child to full term then killing it. There is some evidence human may already change gender ratios this way naturally. $\endgroup$ – John Jan 16 at 23:03
  • $\begingroup$ @John - That sounds interesting. Do you have any links to this evidence? $\endgroup$ – chasly from UK Jan 16 at 23:10
  • $\begingroup$ two different proposed mechanisms ncbi.nlm.nih.gov/pubmed/2199736 and ncbi.nlm.nih.gov/pubmed/24886317 The latter is more well studied in animals $\endgroup$ – John Jan 17 at 1:19
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In the animal kingdom, some species of reptiles, amphibians, and I think fish can do this by influencing the temperature of the environment in which their eggs develop.

Another possibility might be specific chemical factors in the food triggering the creation of mostly male or mostly female sperm cells.

Neither of which of course works with actual human beings, but for your fantasy creatures either or both might be a factor.

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It's kinda possible. The deciding factor for the sex of the baby will be the chromossome in the male spermatoziod, each espermatozoid have only one sexual chromosome. Some studies points that the spermatozoid with the 'x' chromossome are slower but more resistent than the 'y' spermatozoid which is faster but less resistent, that means if a couple has sex 5 or 4 days before the ovulation the 'x' ones will have more chance of fecuding, if they have sex in the ovulation period, the 'y' one have more chances being faster, the acid of the vagine can also play part on this, a more acid vagina can led to the death of the y spermatozoid, choosing some types of food can change the PH of the vagina helping this process.

In summary, if you want a boy, have sex on ovulation day and eat fruits an vegetables and less meat. If you want a girl, have sex before the ovulation and eat a lot of red meat.

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