Why does purpose in life predict mortality in older adults?

Abstract

SummaryBackgroundPrevious work documents a strong association between a higher sense of life purpose and lower all-cause mortality risk even when controlling for baseline health and proposes that life purpose intervention may provide a low-cost lever to improve health and longevity. Causation, however, is less clear—lower purpose may cause poorer health and decreased longevity, or poorer health may cause decreased longevity and lower purpose. We examine the extent that (1) more comprehensive health metrics and (2) horizon mitigate or strengthen the relation between purpose and mortality risk to better understand causation.MethodsProspective cohort sample of 8 425 individuals aged 50 and older who were eligible to participate in the 2006 Health and Retirement Study Psychosocial and Lifestyle questionnaire. Individuals were followed for three subsequent four-year periods: 2006-2010, 2010-2014, and 2014-2018. A total of 1 597 individuals were excluded in the initial four-year period due to lack of follow up, sample weights, or covariates leaving an initial sample of 6 828 individuals. For the second and third four-year periods, an additional 168 and 349 respondents were lost to follow up, respectively. Cox models were estimated to examine the relation between life purpose and mortality for three horizons (years 1-4, 5-8, and 9-12) with more comprehensive measures of current health. Covariates included age, sex, education, race, marital status, smoking status, exercise, alcohol, BMI, and functional health score.FindingsThe relation between life purpose and mortality was substantially attenuated or disappeared at longer horizons or when using more comprehensive measures of current health.InterpretationMuch of the documented relation between life purpose and longevity arises from poor health causing higher mortality risk and lower purpose (i.e., reverse causation). As a result, life purpose intervention is likely to be less effective than the previous evidence suggests.FundingNone.Research in contextEvidence before this studyWe searched PubMed and Google Scholar with no language or date restriction for the term “life purpose” and found four comprehensive reviews of the life purpose or psychological well-being (which included life purpose in the set of psychological well-being metrics) literatures in the last three years and a 2016 meta-analysis of the relation between life purpose and mortality. Although acknowledging it is possible that reverse causation plays a role in linking life purpose levels to subsequent morbidity and mortality, the prevalent view appears to be that even when controlling for current health levels, higher life purpose causes behavioral, biological, or stress buffering changes that, in turn, cause lower future morbidity and mortality.Added value of this studyWe demonstrate that the relation between life purpose levels and mortality is substantially attenuated or eliminated when better controlling for current health or focusing on a longer horizon. Both results suggest that the relation between life purpose levels and future mortality risk primarily arises from life purpose proxying for current health levels. The evidence suggests poorer health causes lower life purpose rather than lower life purpose causing poorer health.Implications of all the available evidenceAlthough life purpose intervention—either at the provider level or in public policy—may have benefits, it is unlikely to cause greater longevity.