The difference in heterogeneity between the US and the other countries has probably increased from 1950 to today. But I also suspect that some of this difference in life-expectancy between the US and the 14 others is that it might be easier to implement preventive health care measures that reach the whole population in societies that have more comprehensive health care insurance by default.
It would be really interesting to see data comparing the healthiest people in the US and other similarly developed countries.
Yes. But I'm not sure I follow here, both Sweden and the US use the same criteria to report a live birth (All live births are reported) and seem to use the same definition of Infant mortality rate, (Death before first birthday). I fail to see how reporting differences explain the difference in infant mortality between the two countries. Of course, compared to countries that use extra conditions for live births (weight limits etc.) the US would get a higher rate of infant mortality due to having more laxed conditions for what constitutes a live birth.
This report[1] claims that the difference in reporting child mortality isn't the chief cause of the difference in infant mortality between the U.S and similarly developed countries.
Instead the claim that most of this difference is due to the high percentage of pre-term births in the US compared to other developed countries.
The difference between letters and full articles is described here [1]. I don't think that one should view the length of the paper or whether it's published as a Letter or an article as a marker for its scientific importance.
For instance, The paper that Watson & Crick published that described the structure of DNA was only slightly longer than a single page [2].