Page "Low birth-weight paradox" Paragraph 7

Additional support for the hypothesis that birth weight and mortality can be acted on independently came from the analysis of birth data from Colorado: compared with the birth weight distribution in the US as a whole, the distribution curve in Colorado is also shifted to lower weights.

The overall child mortality of Colorado children is the same as that for US children however, and if one corrects for the lower weights as above, one finds that babies of a given ( corrected ) weight are just as likely to die, whether they are from Colorado or not.

The likely explanation here is that the higher altitude of Colorado affects birth weight, but not mortality.