# Human sex ratios

1. Jan 31, 2010

### Malcolm_Sex

If there is a 50/50 percent chance that offspring could be either a male or a female (at least thats what my bio teacher tells me) then what are the chances that the male/female human population is (for the most part) split 50/50?

2. Jan 31, 2010

### zomgwtf

Re: Yo gods knowledge me dis right quick

That would be a great question for a statistician and AFAIK it would be dependant on many different variables.

I think using purely the probabilities something called Poisson distribution would play a large role. In determining how likely it would be for the entire population to be split at any given time in the future by 50/50. I'm not sure however since I am not a statistician. You could send a PM to the member sylas and I'm sure he could explain to you what your question is asking.

The last time I heard anything about sex ratios the estimation for secondary sex ratio was around 107 boys for every 100 girls. Which would I believe be about 52/48... not that far off 50/50.

EDIT: if you are actually interested in human sex ratios and why it would be 1:1 you could start off by reading about this:
http://en.wikipedia.org/wiki/Fisher's_principle
and
http://en.wikipedia.org/wiki/Evolutionarily_stable_strategy

Last edited by a moderator: Jan 31, 2010
3. Jan 31, 2010

### Malcolm_Sex

Re: Yo gods knowledge me dis right quick

It was something that occured to me as a major oversight when the teacher explained it in what was probably not the best way. The fisher's principle link is exactly what I was asking myself. Never done probabilities/statistics though so any difficult concepts I am probably not going to get.

Last edited by a moderator: Jan 31, 2010
4. Feb 1, 2010

### eachus

Re: Yo gods knowledge me dis right quick

Some parts of math are hard, in fact calculus is Latin for hard. ;-) But it is usually easy to explain the results of calculus. The theory behind most statistics depends not just on calculus, but even more difficult math. (Queuing theory IMHO is the worst, solving infinite sets of integrodifferential equations using multiple types of transforms, as homework problems? Ugh!)

In this case, the overabundance of boys in human populations falls naturally out of competition between tribes, and for that matter, the whole idea of tribes in the first place. For a tribe (or other group) maximizing the number of descendants occurs with more boys than girls. But within the tribe, the individual parental bias is toward an equal balance.

If you dig a bit deeper, you find that when warriors are needed, human sexual dynamics favors the birth of boys. However, in a tribe with one dominant male, girls are favored. (Think harems.) This mechanism is not genetic as such. Frequent intercourse by women favors boys. If you are looking to choose the sex of your next child, there are books on how to select for either boys or girls. On the other hand, the one family one child campaign enforced during China's Cultural Revolution resulted in an excess of boys, and the effects of that are still being felt today.

In prehistorical times, and in some tribal cultures that survived into historical times, tribes came and went, but successful tribes were those whose descendants flourished. You might think that the Fisher principle is enough, but human genetics doesn't. The interactions within a tribe favor, not the existence of the tribe as such, but the survival of its children. When times are hard, boys can be forced out of the tribe in hopes they will join more successful tribes (and contribute their genes). If the famine, or whatever, ends, the number of children in the next generation is not reduced.

Later when monarchy became commonplace, smaller nations became aggregated through the same effects. If one king has only daughters, and a neighboring king has only sons, merging kingdoms does not require conquest. Even if you assume that kings without sons occur at random--and our ancestors didn't--over hundreds of generations, this fact significantly contributes to the growth of nations. In the last thousand years or so, of course, wars have been a much more important part of nation building--and destruction.

So it is all very complicated--overall a balance between the sexes is favored, and at the lowest level, the family, the same. In between, in human cultures there is a swirl of temporary biases in one direction or the other.