Why Do We Have More Boys Than Girls?

 

Video Transcript

Most human parents-to-be assume that the sex
of their child comes down to a flip of the chromosomal coin, with an equal chance of
conceiving a boy or girl. But in reality, the odds aren’t even – for every 100 girls
born, the world gains about 106 baby boys. Currently, that skewed sex ratio comes out
to roughly 10 million more baby boys than girls born worldwide each year. There are
some countries where human meddling stacks the odds even higher in favor of boys, but
that doesn’t explain the fixed odds everywhere else. The intrinsic boy/girl ratio is rigged
by Mother Nature. In fact, it’s even more rigged than birth
rates show – human conception results in about 150 male zygotes for every 100 females!
But there’s a rather tragic reason for this big biological boy-bias early on: male fetuses
are much more likely to be miscarried or stillborn than female fetuses, and boys that do make
out of the womb suffer more fatal diseases, take more mortal risks, and fall prey to more
violence than girls. So by the time kids grow up and reach baby-making age, the ratio of
males to females is just about 1 to 1. But the likelihood of a boy even making it
to birth is also influenced by his mom’s living conditions during pregnancy. For example,
when a massive famine struck China in the early 1960s, the relative likelihood of having
a son suddenly dropped – until the famine ended. And male Americans born to billionaires
seem to have higher than average odds of fathering sons. Somehow, female biology suppresses boys’
survival in the womb during tough times and boosts it when times are good. We’ve seen the same pattern in other mammals,
too: when resources are scarce, mothers give birth to fewer males than normal; when resources
are plentiful, they bear more. The best explanation we have for this has
to do with sex — the other kind. In biological terms, the whole goal of copulation is to
reproduce — to pass down your genes to someone who will someday pass them on again. Female
offspring are almost guaranteed to reproduce, famine or no famine, because male mammals
are pretty much always willing to mate. Males, on the other hand, have to compete for mating
privileges – a well-nurtured hunk has a good chance of mating with lots of females, while
a male weakened by famine might not score at all. So male offspring are a bigger risk in general
– at all stages they’re more likely to die, and even if they live they might not
reproduce. But when times are good, boys potential to father lots and lots of babies make them
a biological risk worth taking.

Video Description

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References

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Chahnazarian, A. (1988). Determinants of the sex ratio at birth: Review of recent literature. Social Biology 35 (3–4): 214–235. http://www.tandfonline.com/doi/abs/10.1080/19485565.1988.9988703?journalCode=hsbi20#.U-5oSPldWRM

Coale, A.J. (1991). Excess female mortality and the balance of the sexes in the population: An estimate of the number of “missing females.” Population and Development Review 17(3): 517-523.http://www.jstor.org/discover/10.2307/1971953?uid=3739824&uid=2&uid=4&uid=3739256&sid=21104121507601

Clutton-Brock, T. H., Albon, S. D., and Guinness, F. E. (1986). Great expectations: dominance, breeding success and offspring sex ratios in red deer. Animal Behavior 34: 460-471. http://www.kora.ch/malme/05_library/5_1_publications/C/Clutton-Brock_et_al_1986_Dominance_breeding_success_and_offspring_sex_ratios_in_red_deer.pdf

Kalben B. (2000). Why men die younger: causes of mortality differences by sex. North American Actuarial Journal 4(4): 83–111. https://www.soa.org/news-and-publications/publications/other-publications/monographs/m-li01-1-toc.aspx

Schnettler, S. (2013). Revisiting a Sample of U.S. Billionaires: How Sample Selection and Timing of Maternal Condition Influence Findings on the Trivers-Willard Effect. PLoS ONE 8(2): e57446. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057446

Song, S. (2012). Does famine influence sex ratio at birth? Evidence from the 1959–1961 Great Leap Forward Famine in China. Proceedings of the Royal Society B 279(1739):2883-2890. http://rspb.royalsocietypublishing.org/content/early/2012/03/16/rspb.2012.0320.full

Thogerson C.M., Brady, C.M., Howard, R.D., Mason, G.J., Pajor, E.A., Vicino, G.A., and Garner, J.P. (2013) Winning the genetic lottery: biasing birth sex ratio results in more grandchildren. PLoS One 8(7):e67867. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0067867

Trivers, R.L., and Willard, D.E. (1973). Natural selection of parental ability to vary the sex ratio of offspring. Science 179: 90-92. http://www.sciencemag.org/content/179/4068/90.abstract

Photo credits:

Elephant Seal 1
Photo by: Roy Mangersnes
http://roywildphoto.blogspot.com/2008/11/elephant-seals.html

Elephant Seal 2
Photo by: Mike Baird
https://www.flickr.com/photos/mikebaird/11830159673/

White tailed deer
Photo by: Public Domain
http://en.wikipedia.org/wiki/File:White-tailed_deer.jpg

Key Deer
Photo by: IanarÈ SÈvi
http://commons.wikimedia.org/wiki/File:Odocoileus_virginianus_clavium_fawn.jpg

Male Lion
Photo by: Wikimedia user Robek
http://commons.wikimedia.org/wiki/File:Pair_of_lions_v2.jpg

Baby Lion
Photo by: Flickr user fortherock
https://www.flickr.com/photos/fortherock/3897872187/

Brad Pitt
Photo by: Foreign and Commonwealth Office
http://www.flickr.com/photos/foreignoffice/14217374639/

Kids
Photo by: Barbara Hobbs
http://commons.wikimedia.org/wiki/File:Upclose_of_kids_-_Alberti_Flea_Circus,_MerleFest_2013.jpg

Author: dhobson

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