World Population imbalance between the sexes

[kandrathe]

The one hypothesis I can see for why western culture has so many females to males could be attributed to wars, ...

Actually, there is a natural propensity for females in human reproduction. I think the odds are like 41 to 40 for a zygote to be female. Makes sense in a genetic sense. To maintain a population, it takes females. A male can engender many children, a female relatively few. And giving birth is the biggest differential danger during the fertile years (which, until comparatively recently was the whole of the life expectancy).

I understand there are also more lethal genetic issues with the Y chromosome resulting in cell death after fertilization. "Thus, genetic drift is an exceptionally strong force acting upon the Y chromosome. Through sheer random assortment, an adult male may never pass on his Y chromosome if he only has female offspring. Thus, although a male may have a well adapted Y chromosome free of excessive mutation, it may never make it in to the next gene pool. The repeat random loss of well-adapted Y chromosomes, coupled with the tendency of the Y chromosome to evolve to have more deleterious mutations rather than less for reasons described above, contributes to the species-wide degeneration of Y chromosomes through Muller's ratchet. // Primate Y chromosomes, including in humans, have degenerated so much that primates will also evolve new sex determination systems relatively soon, in about 14 million years in humans.-- Wikipedia"

And... Yes, men die at a higher rate for many reasons (Male vs Female Mortality Rates). One idea that surprised me from this article was the higher mortality due to disease and parasites related to the immuno-suppressive effect of testosterone. I've seen studies where women are more attracted to the scent (on t-shirts) of men who have a complimentary immune system. In that article, they also discuss how male risk taking behavior affords more mating opportunities, thus perpetuating that trait (fight or flight). Women are most important for ensuring the survival of their offspring, so women tend to be more risk averse (tend and befriend). Beyond physical capability, I think innately we understand that men are more expendable, and this is why we dedicate men to the more dangerous tasks in our society.

Is anyone else somewhat skeptical about the 9 Billion population figure?

I am. There are many assumptions made to get that (or any other figure) for future populations. Presently, most of the industrial nations would be having a population reduction if it weren't for immigration. China, as you pointed out, is already in a population draw down mode (although I doubt it will be as extreme as your analysis makes it). India and Africa are the jokers in that deck. As long as the culture is based on the concept of high infant and child mortality (thus making having a lot of children a benefit) but the technology is developing towards that of industrial countries (where having more than two or so children causes a population explosion) there will be problems.

A combination of local laws, a strong propaganda campaign, and tying foreign aid to reasonable population control could make the situation go the other way, and a population of 4 to 5 billion by mid century is not impossible.

China has taken steps to curb their population explosion, however they now are looking at a time when they will have many more elderly than those capable of supporting them. India is the next large population producer than has no reins on their uncontrolled growth.

...the Yellowstone Caldera might erupt taking out a large number of the population in the US and Canada immediately with a large amount of people dying due to a volcanic winter due to the ash cloud created much like Krakatoa, and other aspects).

We can count on severe natural events that will adversely affect people and their resources, but truly cataclysmic events are fortunately rare. Predicting a supervolcanic eruption in the next 50 years is very speculative. The last VEI8 (Volcanic Explosivity Index) eruption occurred ~26,500 years ago, and the last VEI7 was Tambora in 1815. The last VEI6 was Mount Pinatubo in 1991. Mount St. Helens was a VEI5. The last time Yellowstone went supercritial was 640,000 years ago. It might be due to blow it's top, but it might take a few more thousand years.