Does having a sense of purpose increase longevity? There is evidence that people who retire early tend to die earlier than people who do not. One interpretation of this finding is that having a sense of purpose improves psychological health and that psychological health is linked to physical health. 

What is the evidence for the validity or invalidity of this interpretation? (I am equally interested in the validity of this interpretation with respect to early retirement and with respect to other measures/correlates of purpose.)


  http://longevity.about.com/od/healthyagingandlongevity/a/retirement.htm
I did a little search and found an article by Moen (1996) which seems relevant. 
Moen summarised the existing evidence as follows:


  Poor physical health is frequently a reason for retiring (Anderson &amp;
  Burkhauser, 1985; Bound, 1991; Chirikos &amp; Nestel, 1989; Palmore,
  Burchett, Filenbaum, George, &amp; Wallman, 1985), leading to a view of a
  unidirectional link between health and retirement (see Figure 1). In
  support of this one-way model, most of the existing research
  literature has shown that retirement per se is not directly related to
  early mortality or health decline. However, there is some evidence
  that being retired affects psychological symptoms (Bosse, Aldwin,
  Levenson, Spiro, &amp; Mroczek, 1993; Ekerdt, 1987). Other studies show
  that retired individuals sometimes report improvements in their
  (subjectively rated) health status following retirement (Ekerdt,
  Bosse, &amp; Locastro, 1983). Most early research on retirement suggests
  that being retired has no deleterious effects on either physical or
  psychological health; most retirees say they are satisfied with their
  retirement and some even report better health (Atchley, 1976; Barfield
  &amp; Morgan, 1968; McGoldrick &amp; Cooper, 1989; Streib &amp; Schneider, 1971).
  However, research on the effects of the loss of the work role through
  retirement often ignores the complexity of the retirement transition,
  including issues of timing, previous health, and quality of the
  employment experience (McGoldrick &amp; Cooper, 1988). George (1993)
  pointed out that most research on stress and well-being fails to
  consider either the social contexts or the timing of potentially
  stressful life experiences, such as retirement.


References


Anderson, K. H., &amp; Burkhauser, R. V. (1985). The
retirement-health nexus: A new measure of an old puzzle.
The Journal of Human Resources, 20, 315-330.
Atchley, R. (1976). The sociology of retirement. New York:
Schenkman.
Barfield, R., &amp; Morgan, J. N. (1968). Early retirement: The
decision and the experiences. Ann Arbor, MI: Institute for
Social Research.
Bound, J. (1991). Self-reported objective measures of health
in retirement models. The Journal of Human Resources,
26, 108-137.
Bosse,  R., Aldwin, C. M., Levenson, M. R., Spiro, A., &amp;
Mroczek, D. K. (1993). Change in social support after
retirement: Longitudinal findings from the normative
aging study. Journal of Gerontology, 48, 210-217.
Chirikos, T. N., &amp; Nestel, G. (1989). Occupation, impaired
health, and the functional capacity of men to continue
working. Research on Aging, 11, 174-205.
Ekerdt, D. J. (1987). Why the notion persists that retirement
harms health. Gerontologist, 27, 454-^57.
Ekerdt, D. J., Bosse, R., &amp; Locastro, J. S. (1983). Claims that
retirement improves health. Journal of Gerontology, 38,
231-236.
George, L. K. (1993). Sociological perspectives on life
transitions. Annual Review of Sociology, 19, 353-373.
McGoldrick, A., &amp; Cooper, C. (1988). Early retirement.
Chichester, England: Gower
Moen, P. (1996). A Life Course Perspective on Retirement, Gender, and Well-Being. Journal of Occupational Health Psychology, 1, 131-144.
Palmore, E. B., Burchett, B. M., Filenbaum, G. G., George,
L. K., &amp; Wallman, L. M. (1985). Retirement: Causes and
consequences. New York: Springer.
Streib, G. F, &amp; Schneider, C. J. (1971). Retirement in
American society. Ithaca, NY: Cornell University Press.