The currently accepted value of the proportion of female births in large European-race populations is 0.485 (Bayesian Data Analysis, 3rd edition, Gelman et al.).
Do we know why the probability of being born male or female deviates from 0.5?
Thank you for your insights
A clear treatment on the topic of sex ratios can be found in this book;
Charnov, E.L. 1982. The Theory of Sex Allocation. Princeton University Press.
My understanding is that this fact is not explained for sure yet. The folk theory is about speed of swim for male and female sperm (YX - lighter and XX -havier).
You could check a small-talk disscussion in a regular newspaper here:
https://www.bbc.com/news/health-46597323
Now a day one of the sex would be born in higher proportion than the other sex in livestock and one of the reason could be season. Read the undermentioned paper
https://www.ncbi.nlm.nih.gov/pubmed/24908336
There were nice theory about it. See Geodakyan V. A. "Evolutionary theoty of sex"
In developing countries like India, female foeticides are very prevalent. Women are forced to go for abortions just because they are carrying female fetuses for cultural reasons! That is screwing up the sex ratio.
I believe that the xplanation is not based on genetic causes, probably is related to the fact that more boys are lost early in life.
It is a good question. I think, we should consider the influence of oral contraceptives and related practices in birth control, racial admixtures and environmental factors. People working in population genetics often suggest that the genetic background of the population or family decides male or female predominance in the given locality. However, we cannot pinpoint one factor for the increase of one gender.
Some suggested answers, quoting familial preference in India or China, as well as the use of contraceptives etc, has no relevance to this issue. 105 boys to 100 girles ratio of birth rate is true for the Britain and true for old Britain too, when no such drugs were avalable.
There are most likely more than one cause, but the BPA in most plastics and liners of tin cans increase estrogen levels.
This effect has nothing to do with mordern time factors, like BPA. It is documented since time when chirch records of birth begun.
"BPA is a synthetic estrogen called a xenoestrogen, which mimics the way the female hormone estrogen acts." So, I doubt it was a cause in the past, but it probably is today.
In India it is in the range of 933-980 girls per 1,000 boys. WHO and several world bodies say this is definitely because of female foeticide based on cultural conditions. Contrary to what some say, it definitely has relevance in this part of the world.
Is it shown that higher level of estrogen in water will cause more birth of girls? Do we have to assume that higher level of totesteron in water will cause higher probability of boys to be born? Any papers I could read about that phenomenon?
There NO doubt that effect is even higher in India and China, and reason for that is clear. What is discussed here, I think, is more general phenomenon, which is true for humans at least, and does NOT depends on human behevior in choice of babies sex.
It is my understanding that this is as Nature intended. More males are born because more males die in their youth compared to females. In ancient times, males were the providers of food from the wild which necessitated more risky lifestyles. In contemporary life risk taking is still more prevalent in the male population, just that it isn’t much related to food provision.
Females have needed to remain close to home to care for their young and coincidently worked with food sources close to home that aren’t dangerous. Perhaps in another million years the 0.5 will apply to both sexes, but I doubt it due to the hormonal differences.
https://www.washingtonpost.com/news/worldviews/wp/2015/08/19/see-where-women-outnumber-men-around-the-world-and-why/?noredirect=on&utm_term=.aa3cf8803f15
See where women outnumber men around the world (and why)
https://www.bbc.com/news/health-46597323
Why are more boys than girls born every single year?
Because of sex reassignment surgery (SRS) to prevent hermaphroditism, which is mandatory surgical procedure in many countries, futanarian women cannot reproduce with their own species of women and so the proportion of male brains, that is, male births, is higher ratiologically: 'The number of a man is the number of the beast and his number is six hundred three score and six.' (Rev: 13. 8) That is, 66.6% with 33.3% hermaphrodites missing from the equation.
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