It is known that greater health throughout life, greater wealth, greater social status, and greater intelligence are all associated with greater life expectancy. Untangling the nature of these linked correlations is ever a challenge, however, since they all associate with one another as well. There are any number of plausible explanations as to why the wealthy or the more intelligent live longer, and some interesting speculation besides, such as the association of intelligence with physical robustness, but rarely is there any way to prove these explanations true in the data obtained from population studies. Correlations are what is obtained, and it is then usually a matter of retreating to animal studies where it is possible to structure the work to prove causation - but of course this is somewhere between hard and impossible to achieve for intelligence and social status.
This paper reminds us of the tendency for age-matched cohorts to become on average healthier, wealthier, and wiser over time. This isn't a matter of self-improvement, though any of us can work on that, but occurs because those who were not comparatively healthy, wealthy, and wise are more likely to be dead already:
The gradual changes in cohort composition that occur as a result of selective mortality processes are of interest to all aging research. We present the first illustration of changes in the distribution of specific cohort characteristics that arise purely as a result of selective mortality. We use data on health, wealth, education, and other covariates from two cohorts (the AHEAD cohort, born 1900-23 and the HRS cohort, born 1931-41) included in the Health and Retirement Survey, a nationally representative panel study of older Americans spanning nearly two decades (N=14,466). We calculate sample statistics for the surviving cohort at each wave. Repeatedly using only baseline information for these calculations so that there are no changes at the individual level (what changes is the set of surviving respondents at each specific wave), we obtain a demonstration of the impact of mortality selection on the cohort characteristics.
We find substantial changes in the distribution of all examined characteristics across the nine survey waves. For instance, the median wealth increases from about $90,000 to $130,000 and the number of chronic conditions declines from 1.5 to 1 in the AHEAD cohort. The mortality selection process changes the composition of older cohorts considerably, such that researchers focusing on the oldest old need to be aware of the highly select groups they are observing, and interpret their conclusions accordingly.