Today is the 7 Billion Person Day - the growth of our living population beyond 7 Billion. October 12, 1999 was considered the 6 Billion Day. In just the lifetime of Ellen (12 years) we have added a billion people (births net of deaths). The US is about 300 million of this - 4.3% but has 22% of the World GDP (note GDP varies with population so the percentages have some dependency).
The current book I am reading is "Gross National Happiness" by Arthur C. Brooks. I suspect that the US does not have 22% of the Worlds Gross National Happiness. In fact, it is well documented and proved that income and wealth does not assure happiness.
Most intriguing is Brook's chapter on whether Happiness is a Family Value. He addresses the scientific data surrounding whether having children increases or decreases happiness for the parents. Interestingly it is not a black and white answer - however the bottomline is that over time and across all populations the happiness cost benefit favors procreating.
I guess there is a reason we have 7 billion people - all in the pursuit of happiness.